Chapter 3 - Amount of substance Flashcards
Why are chemicals usually measured by mass or volume?
Reactions occur on the atomic scale, so chemists need a way to convert measured mass or volume into the number of particles involved in the reaction.
What is the ‘amount of substance’? What is it represented by? Why is it measured in?
It is a measure used to count the number of particles in a substance, represented by n, and measured in moles (mol).
What is a mole?
A mole is the amount of substance that contains 6.02 × 10²³ particles. This number is known as the Avogadro constant.
What is the Avogadro constant?
6.02 × 10²³ particles.
Why is the Avogadro constant set at
6.02 × 10^23?
It is directly linked to the mass of carbon-12.
Example: 12 grams of carbon-12 contains 6.02 × 10^23 atoms.
How can you find the mass of one mole of atoms?
The mass of 1 mole of an element is its relative atomic mass in grams.
Example:
1 mole of carbon = 12.0g
1 mole of magnesium = 24.3g
Why is the mole an important concept in chemistry?
It provides an easy way to count atoms or particles by simply measuring the mass.
Does the mole only refer to atoms?
No, it can refer to anything, such as atoms, molecules, or ions. It’s important to specify the formula or name of the substance being measured.
What is molar mass?
Molar mass is the mass per mole of a substance, measured in g/mol
What is the molar mass of Carbon?
12.0 g/mol
What is the molar mass of NO2?
14.0 + (16.0 x 2) = 46.0g/mol
What is the formula linking the amount of substance, mass, and molar mass? Give it as the symbol equation as well.
Amount of substance = Mass ÷ Molar Mass
n = m ÷ M
What is the unit used to measure the Amount of substance?
mol
What is the unit used to measure mass?
g
What is the unit used to measure Molar Mass?
g/mol
PRACTICE QUESTION:
Calculate the amount of substance, in moles, in 96.0g of carbon, C.
n = m ÷ M
n = 96.0 ÷ 12.0
n = 8.0 mol
PRACTICE QUESTION:
Calculate the mass, in g, of 0.050mol of NO2.
Rearrange: n = m ÷ M
Rearranged formula: m = n x M
m = 0.050 x 46.0
m = 2.3g
PRACTICE QUESTION:
Calculate the molar mass when 2.65g contains 0.025mol of a substance.
Rearrange: n = m ÷ M
Rearranged formula: M = m ÷ n
M = 2.65 ÷ 0.025
M = 106.0 gmol-1
What is a molecular formula?
The molecular formula is the exact number of atoms of each element in a molecule.
What is an empirical formula?
The empirical formula is the simplest whole-number ratio of atoms of each element in a compound.
Why is the empirical formula important?
It is important for substances that do not exist as molecules, such as metals, some non-metals (e.g., carbon, silicon), and ionic compounds (e.g., sodium chloride).
Why can’t the actual number of atoms or ions in giant crystalline structures be used?
The actual numbers would run into billions and vary depending on the crystal size, making them impractical.
How is the empirical formula determined for giant crystalline structures?
It represents the ratio of atoms or ions in the structure, which is always the same.
How do you convert between molecular and empirical formulas?
Divide the molecular formula by the greatest common factor of the subscripts.
If the molecular formula is N2O4, what is the empirical formula?
NO2
If the molecular formula is H2O, what is the empirical formula?
H2O
If the molecular formula is C2H6, what is the empirical formula?
CH3
If the molecular formula is P4O6, what is the empirical formula?
P2O3
If the molecular formula is C9H12O3, what is the empirical formula?
C3H4O
Why are two terms needed for relative mass?
- Relative molecular mass (Mr): For simple molecules like water and carbon dioxide.
- Relative formula mass (Mr): For giant crystalline structures like ionic compounds.
What is relative molecular mass (Mr)?
It compares the mass of a molecule with the mass of an atom of carbon-12.
How is relative molecular mass calculated?
By adding the relative atomic masses of the elements in the molecule.
What is the relative molecular mass of: H2O?
Mr(H2O) = (1.0x2) + 16.0 = 18.0
What is the relative molecular mass of: CH4?
Mr(CH4) = 12.0 + (1.0x4) = 16.0
What is the relative molecular mass of: C6H12O6?
Mr(C6H12O6) = (12.0x6) + (1.0x12) + (16.0x6) = 180.0
What is relative formula mass?
It compares the mass of the formula unit with the mass of an atom of carbon-12.
How is relative formula mass calculated?
By adding the relative atomic masses of the elements in the empirical formula.
What is the relative formula mass of: NaCl?
Mr(NaCl) = 23.0 + 35.5 = 58.5
What is the relative formula mass of: Ca(NO3)2?
Mr(Ca(NO3)2) = 40.1 + ((14.0+(16.0x3))x2) = 164.1
How can the formula of an ionic compound be predicted?
From the known ions present in the compound.
How can you determine the formula of an ionic compound if the ions in a compound are unknown?
The formula can be determined through experiments analyzing the chemical composition of the substance (a process known as analysis).
PRACTICE QUESTION:
In an experiment, 1.203g of calcium combines with 2.13g of chlorine to form a compound. Calculate the empirical formula.
[Ar: Ca=40.1; Cl=35.5]
Step 1: Convert the mass into moles of atoms using n = m ÷ M
n(Ca) = 1.203 ÷ 40.1 = 0.030 mol
n(Cl) = 2.13 ÷ 35.5 = 0.060 mol
Step 2: To find the smallest whole-number ratio, divide by the smallest whole number.
na(Ca):n(Cl) = 0.030÷0.030 : 0.060÷0.030 = 1:2
Step 3: Write the empirical formula: CaCl2
PRACTICE QUESTION:
Chemical analysis of a compound gave the percentage composition by mass: C=40.00%; H=6.67%; O=53.33%. The relative molecular mass of the compound is 180.0
Calculate the molecular formula.
[Ar: C=12.0; H=1.0; O=16.0]
Step 1: Convert % by mass into moles of atoms using n = m ÷ M
n(C) = 40.00 ÷ 12.0 = 3.33 mol
n(H) = 6.67 ÷ 1.0 = 6.67 mol
n(O) = 53.33 ÷ 16.0 = 3.33 mol
Step 2: Find smallest whole-number ratio and empirical formula.
n(C):n(H):n(O) = 3.33÷3.33 : 6.67÷3.33 : 3.33÷3.33 = 1:2:1
Empirical formula = CH2O
Step 3: Find the relative mass of the empirical formula CH2O: 12.0 + (1.0x2) + 16.0 = 30.0
Step 4: Find the number of CH2O units in one molecule: 180 ÷ 30.0 = 6
Step 5: Write the molecule formula: CH2O x 6 = C6H12O6
What are hydrated salts?
Hydrated salts are crystalline compounds that contain water molecules as part of their crystalline structure.
What is the water in hydrated salts called?
It is called water of crystallization.
What happens when hydrated copper(II) sulfate is heated?
The bonds holding the water molecules are broken, and the water is driven off, leaving behind anhydrous copper(II) sulfate (a white powder).
What is the chemical equation for the removal of water from hydrated copper(II) sulfate?
CuSO4*5H2O(s) (hydrated) –> CuSO4(s) (anhydrous)+ 5H2O(l)
What happens to the crystalline structure of copper(II) sulfate when the water is removed?
The crystalline structure is lost, and the compound becomes a white powder.
Why is it difficult to remove all traces of water from hydrated copper(II) sulfate?
It is difficult because some water may remain trapped inside the crystals, and the anhydrous copper(II) sulfate retains a very pale blue color.
How can you ensure that all the water has been removed from hydrated salts?
Heat the sample until it reaches a constant mass, meaning the mass of the residue no longer changes with further heating.
What are two main assumptions made in experiments to determine the formula of hydrated salts?
- All the water has been lost.
- No further decomposition occurs.
Why can it be challenging to determine when all the water has been removed?
You can only see the surface of the crystals, so some water may remain trapped inside.
Why does color play a role in determining when water is removed?
If the hydrated and anhydrous forms have different colors, it is easier to judge when all the water has been removed.
What happens if a salt decomposes further during heating?
Some salts, like copper(II) sulfate, can decompose further (e.g., into black copper(II) oxide) if heated too strongly.
Why is further decomposition hard to detect?
If there is no significant color change, it can be difficult to judge whether decomposition has occurred.
Why are liquids and gases measured by volume in chemistry?
Liquids and gases are measured by volume because it allows the amount of substance in moles to be calculated, which provides a way to count particles present.
What are the common units for measuring volume in chemistry?
Cubic centimeter (cm³) or millimeter (ml): 1cm³ = 1ml
Cubic deceimeter (dm³) or litre (L): 1dm³ = 1000cm³ = 1000ml = 1L
Which units should you use when recording volume in practical chemistry work?
Use centimeters cubed (cm³) and decimeters cubed (dm³).
What is the concentration of a solution?
The concentration of a solution is the amount of solute (in moles) dissolved in 1 cubic decimeter (dm³) of solution. It is measured in
moldm^−3.
What does a concentration of 1 mol dm⁻³ mean?
It means 1 mole of solute is dissolved in 1 dm³ (or 1000 cm³) of solution.
What is the equation that links moles, concentration, and volume for a solution?
n = c x V
n = amount of substance (in moles)
c = concentration (in mol dm⁻³)
V = volume (in dm³)
How do you adjust the n = c x V, for volumes measured in cm³?
Divide the volume in cm³ by 1000 to convert it to dm³, giving:
n = c x (V(cm³)÷1000)
Why do you convert between
Concentration is measure in mol dm⁻³, so the volume must also be in dm³ for calculations.
What is a standard solution?
A standard solution is a solution with a known concentration.
PRACTICE QUESTION: Calculate the amount of NaCl, in mol, in 30.0cm³ of a 2.00
n(NaCl) = c x (V(cm³)÷1000)
n(NaCl) = 2.00 x (30.0÷1000)
n(NaCl) = 0.0600 mol
PRACTICE QUESTION:
Calculate the volume of a 0.160 mol dm⁻³ solution that contains 2.35 x 10^⁻³ mol of NaCl.
n(NaCl) = c x (V(cm³)÷1000)
so, V = (1000xn)÷c
V = (1000 x 3.25 x 10^⁻³) ÷ 0.160
V = 20.3cm³
How can you calculate the mass required to prepare a standard solution?
Use your understanding of the mole equation to calculate the mass needed:
m = n x M, where:
m = mass of solute (g)
n = amount of solute (mol)
M = Molar mass of the solute (g mol⁻¹)
How are standard solutions labeled in practical work?
Standard solutions are often labeled with their concentration, such as 1 mol dm⁻³.
How is a standard solution prepared?
- An exact mass of solute is dissolved in a solvent.
- The solution is then made up to an exact volume using a volumetric flask.
What are the units used for mass concentrations?
Mass concentrations are shown in g dm⁻³.
PRACTICE QUESTION:
Calculate the mass of Na2CO3, required to prepare 100cm³ of a 0.250mol dm⁻³ standard solution.
Step 1: First work out the amount in moles required.
n(Na2CO3) = c x (V(cm³)÷1000)
n(Na2CO3) = 0.250 x (100÷1000)
n(Na2CO3) = 0.0250 mol
Step 2: Then work out the molar mass of Na2CO3.
M(Na2CO3) = (23.0 x 2) + 12.0 + (16.0 x 3) = 106.0g mol⁻¹
Step 3: Rearrange n = m ÷ M, to calculate the mass of Na2CO3 required.
m = n x M
m = 0.0250 x 106.0
m = 2.65g
How do you calculate mass concentration from molar concentration?
Use the formula:
m = n x M, where:
m = mass of solute (g)
n = molar concentration (mol dm⁻³)
M = Molar mass (g mol⁻¹)
Why is gas volume easier to measure than gas mass?
Measuring gas mass is difficult, but gas volumes can be easily measured.
What is the relationship between gas volume and the number of gas molecules?
At the same temperature and pressure, equal volumes of different gases contain the same number of molecules.
What is molar volume?
Molar volume is the volume per mole of gas molecules at a stated temperature and pressure.
What is the molar volume at room temperature and pressure (RTP)?
At RTP (20°C and 101 kPa or 1 atm)
What is 1mol of gas equal to, in volume, at RTP?
1molofgas = 24.0dm³ = 24,000cm³.
What is the equation to convert gas volume into moles?
Amount (n) is equal to the volume (V) divided by the molar gas volume.
What is the ideal gas equation?
The ideal gas equation is:
pV=nRT
What does each variable in the ideal gas equation represent?
p: Pressure, measured in Pascals (Pa).
V: Volume, measured in meters cubed (m³).
n: Amount of gas molecules, measured in moles (mol).
R: Ideal gas constant, always 8.314Jmol⁻¹K⁻¹.
T: Temperature, measured in Kelvin (K)
What are the necessary unit conversions for the ideal gas equation?
cm³ → m³ = x10^-6
dm³ → m³ = x10^-3
°C → K = +273
kPa → Pa = x10^3
QUICK CHECK:
Convert 25°C to Kelvin
T = 25 + 273 = 298K
Why might room temperature and pressure (RTP) not always be accurate?
- RTP (20°C and 101 kPa) is approximate and convenient for experiments.
- At standard temperature (25°C) and atmospheric pressure, the molar gas volume is 23.5 dm³ mol⁻¹.
What assumptions are made about an ideal gas?
- Molecules move in random motion.
- Collisions are elastic (no energy loss).
- Molecules have negligible size compared to the container.
- There are no intermolecular forces between molecules.
Under what conditions do the assumptions of the ideal gas equation break down?
- High pressures: Gas molecules are close together, and their volume becomes significant compared to the container.
- Low temperatures: Molecules move slower, and intermolecular forces may become significant.
What conditions favor the assumptions of the ideal gas equation?
- Low pressure: Molecules are far apart.
- High temperature: Molecules move quickly, reducing the effect of intermolecular forces.
What does the term stoichiometry refer to in a balanced equation?
It refers to the ratio of the amounts (in moles) of each substance involved in the reaction.
In the equation 2H2(g) + O2(g) –> 2H2O(l), what is the stoichiometric ratio?
2 mol of H2: 1 mol of O2: 2 mol of H2O
How do chemists use balanced equations in practice?
- To calculate the quantities of reactants required to produce a desired quantity of product.
- To determine the quantities of products expected from a given amount of reactants.
What are the three steps for solving stoichiometry problems?
- Calculate the amount (in moles) of a known substance.
- Use the stoichiometric ratio to find the moles of the unknown substance.
- Calculate the required unknown information (mass, volume, etc.).
What is theoretical yield?
The maximum possible amount of product from a reaction if all reactants were converted into the desired product.
Why is the actual yield often lower than the theoretical yield?
- The reaction may not go to completion.
- Side reactions may occur.
- Loss of product during purification.
How is percentage yield calculated?
Percentage yield = (Actual yield ÷ Theoretical yield) x 100%
What is the limiting reagent in a reaction?
The reactant that is completely used up first and stops the reaction.
How do you determine the limiting reagent?
Calculate the moles of each reactant and compare them using the stoichiometric ratio from the balanced equation.
In the reaction 2H2(g) + O2(g) –> 2H2O(l), what is the limiting reagent if equal amounts of H2 and O2 are used?
H2 is the limiting reagent because 2 moles of H2 are required for every mole of O2. Half of the O2 will remain unreacted.
What does atom economy measure?
Atom economy measures how efficiently reactants are converted into the desired product.
What are the benefits of reactions with high atom economies?
- They produce a large proportion of desired products with minimal waste.
- They are more sustainable by making better use of natural resources.
How is atom economy calculated?
Atom Economy = (Sum of molar masses of desired products ÷ Sum of molar masses of all products) x 100%
What factors influence the sustainability of a chemical process?
- Atom economy: How efficiently raw materials are used.
- Percentage yield: How much of the theoretical yield is achieved.
- Environmental concerns: E.g., waste products like CO2 contributing to global warming.
- Costs: Availability and cost of starting materials and energy required for the process.
