Module 2 (Quantitative Chemistry) Flashcards
What does the law of conservation of mass state?
It states that matter can be neither created nor destroyed but merely changed from one form to another
What is the significance of the law of conservation of mass in a chemical reaction?
Suggests that mass is conserved in a chemical reaction, thus:
The total mass of products = the total mass of reactants
What is a chemical reaction?
A chemical reaction occurs when bonds are broken, and bonds are made
What is the formula for Density?
D = m/v
Where D= Density (in g mL^-1 or g L^-1, m = mass (in g), v = volume (in mL or L)
What is stoichiometry?
Refers to a quantitative study of formulae and equations
What is the accepted quantity for chemists?
moles (can be written as ‘n’ and ‘mol’)
What is the Relative atomic mass?
The Relative atomic mass (RAM) of an element is the average mass of the atoms present in a naturally occurring element compared with 1/12th of the mass of a carbon-12 atom. Since it is a ratio, there are no units
What is the formula for molecular mass/weight?
Sum of atomic weight or atoms in an average molecule
What is the numerical value of Avogadro’s number?
6.022 * 10^23 atoms/molecules per mole
What is the significance of Avogadro’s number? (What does it mean). Give examples.
It means that for any element, the mass in grams which is numerically equal to the atomic weight contains 6.022 * 10^23 atoms.
I.e. 24.31g of Mg will mean that there are 6.022 * 10^23 atoms of Mg
It also means that for any compound, the mass in grams which is numerically equal to the atomic weight contains 6.022 * 10^23 molecules.
I.e. 58.44g of NaCl (with a atomic weight of 58.44) will have 6.022 * 10^23 NaCl molecules. This means that there are 6.022 * 10^23 sodium atoms and 6.022 * 10^23 chlorine atoms for every molecule (because of the ratio 1:1), and there are a total of 12.044 * 10^23 atoms (in general)
What is the mole?
The mole is a convenient unit that can be converted to grams, number of particles or in the case of a gas, to volume.
What are two ways of looking at a mole?
A number of particles: atoms, ions or molecules (6.022 * 10^23)
A mass: atomic or molecular weight in grams (?)
What is a molar mass?
The mass of a mole of the substance
What is the equation for the number of moles?
n = m / Mw
n= number of moles m= mass Mw = Molar mass/weight
Why do limiting and excess reagents exist?
When reactants are mixed to create a chemical reaction, sometimes they are combined in just the right mole ratio, however, sometimes one is in excess and the other one is limiting (i.e. has less moles than the other).
What is a limiting reagent?
In a chemical formula with both limiting and excess reagents, this is the reactant that will be completely used up.
The amount of product formed is limited by this reagent, since the reaction cannot continue without it
What is an excess reagent?
In a chemical formula with both limiting and excess reagents, the excess reagent is the reactant that is not completely consumed (i.e. has more moles than the other reagent)
What are the 3 steps to solving a limiting reactant problem where you are required to work out the amount of product?
- Calculate the number of moles of each
- Determine which reactant is in excess. and therefore the limiting reactant
- Use the amount of the limiting reactant to work out the amount of product formed.
What is a mole ratio?
A mole ratio is the ratio between the amounts in moles of any two compounds involved in a balanced chemical reaction. Here, the balanced chemical equation provides a comparison of the ratios of the molecules necessary to complete the reaction.
What is an example of the mole ratio using the equation 2H2 + O2 –> 2H2O
Coefficients of H2 = 2
Coefficients of O2 = 1
Coefficients of H2O = 2
Therefore in this reaction, if there is 2 moles of H2, there will be 1 mole of O2 and 2 moles of H2O as a product (because of the ratios)
Similarly, if there are 4 moles of H2, there will be 2 moles of O2 and 4 moles of HO as a product (because of the ratios
How do you find the percentage of mass of an element in a compound?
(mass of the element in 1 mole of the compound/molar mass of the compound) * 100
What is emperical formula?
Simplest whole number ratio of atoms or ions
What is a molecular formula?
Number of atoms in a molecule (how many of each type) I
Explain the differences between emperical and molecular formulae using the example of C2H4
Emperical: CH2 (For every 1 carbon atom, there are 2 hydrogen atoms)
Molecular: C2H4 (2 carbon atoms for every 4 hydrogen atoms)
What is a significant figure?
A significant figure in a physical quantity is the number of digits in the value for the quantity. It tells us the accuracy in which a quantity was measured
What is an example of three and four significant figures
3sf : 5.04
4sf : 26.02
What are the basic rules of significant figures? (I.e. for multiplication and division, addition and subtraction
When quantities are multiplied or divided, the number of significant figures in the answer is equal to the number of significant figures in the least accurate of the input quantities
When quantities are added or subtracted, the answer should have the same number of DECIMAL PLACES (Not sig fig) as the least accurate input quantity
True or false? In values less than 1, zeroes between the decimal point and the first non-zero digit are significant.
False. They aren’t significant
I.e. 0.0054 has 2 sf not 4
Are molarity and concentration the same thing?
Essentially, yes, molarity is just asking for concentration in mol / L, whereas concentration refer to any units.
What is the formula for concentration in g/L
Mass of solute (in g) / Volume of solution (in L)
What is the formula for concentration/molarity in mol/L
C=n/v
Where c = concentration in mol L^-1
n = number of moles
v = volume in L
How to calculate concentration (ppm)
mass of solute (in mg) / mass of solution (in kg)
What is PPM?
Parts per million
How to calculate concentration (ppb)
Mass of solute (in µg) / mass of solution (in kg)
What is µg?
One millionth of a gram 1 * 10^-6
What is ppb?
Parts per billion
How do you calculate percentage by mass (% (w/w))
Percentage by mass describes the mass of solute, measured in grams, present in 100g of the solution
How do you calculate percentage by volume ( %(v/v))
Percentage is based on the volumes of both solute and solution, where the same units are used to record both volumes
How do you calculate percentage mass/volume
Percentage mass/volume describes mass of solute, measured in grams, present in 100mL of solution
What is molarity?
Molarity of a solution is the number of moles of solute per litre of solution
What are 4 properties of a primary standard?
Be obtainable in a very pure form
Not alter during weighing by picking up or losing moisture or reacting with air
It is stable
Have a reasonably high relative formula mass, to minimise weighing errors
What is a primary standard?
In volumetric analysis, it is a substance of sufficiently high purity and stability
What is the purpose of dilutions?
Sometimes, we need to dilute solution from one concentration to a lower one. This is done by measuring out a known volume of the original solution and adding water to make a ne volume. In this process, the number of moles of solute is unchanged, only concentration has decreased
What formula can be used to calculate concentrations and volumes in dilutions, where c1 and v1 are the original concentrations and volume, and c2 and v2 are the new concentrations and volume
c1v1 = c2v2
What is gay-lussac’s law of combining volumes?
p1/t1 = p2/t2
Where p1 = original pressure (in kPa), t1 = original temperature (in kelvins)
Where p2 = changed pressure (in kPa), t2 = changed temperature (in kelvins)
What is Avogadro’s law?
v1 / n1 = v2 / n2
Where v1 = original volume, n1 = original number of moles
Where v2 = changed volume, n2 = changed number of moles
What are the characteristics of gases?
Low density
Fill in the shape of container
Compress easily
Mix together rapidly
What is the relationship between volume and pressure?
The more a gas is compressed, the more the particles collide with each other, and the walls of the container that they are in, therefore exerting a greater force on the walls, this force per unit area is called pressure
What are 2 common units of pressure
Kilopascals (kPa)
Atmospheres (atm)
How many kPa in 1 atm
101.3
What is Dalton’s Law of partial pressure?
In a gaseous mixture, the total pressure is the sum of the partial (individual) pressures of each gas.
P(t) = P(1) + P(2) +P(3)…
Where P(t) = total pressure and P(1), P(2) etc. are the other partial pressures
What did Boyle find out?
That by measuring the pressure and volumes of many samples oof different gases at constant temperature, for a given quantity of gas, the product of its volume and pressure is constant
What did Boyle conclude? (I.e. the formula)
P1V1 = P2V2
Where P1 = Original pressure (in kPa), V1 = Original volume (in L)
Where P2 = changed pressure (in kPa), V2 = changed volume (in L)
What is absolute zero? (Values)
-273.15 degrees Celsius
0 degrees kelvin (absolute zero)
What happens at absolute zero?
Particles stop moving and all disorder happens, nothing can be colder than absolute zero
How do you convert Celsius to kelvins?
Kelvins = Celsius + 273.15
Do you have to use kelvins for all temperatures?
Yes
What is Charles Law? (formula)
V2/T2 = V1/T1
Where V1 = Original volume (in L), T1 = Original temperature (in K)
Where V2 = Changed volume (in L), T2 = changed temperature (in K)
What is the combined gas law?
P1V1/T1 = P2V2/T2
Where, P1 = Original pressure (kPa), V1 = Original volume (L), T1 = Original Temperature (Kelvin)
Where, P2 = Changed pressure (kPa), V2 = Changed volume (L), T2 = Changed Temperature (Kelvin)
What is the ideal gas law?
PV = nRT
Where P = Pressure in kPa V= Volume in L n= number of moles R = Gas constant (8.3145J) T = Temperature in Kelvins
What is the gas constant?
8.3145 J
How many kPa is 1 bar?
100
