C3 - Quantitive Chemistry Flashcards
Define the law of conservation of mass
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
Why do chemical equations always need to be balanced?
So that the mass of the products is equal to the mass of the reactants; law of conservation of mass
If the mass appears to change when a product or reactant is a gas in an NON-ENCLOSED SYSTEM, why is this?
The mass will DECREASE if the reaction vessel is not ENCLOSED as the gas will escape as a waste product
The mass will INCREASE when the gas reacting becomes contained in the reaction vessel
How can a scientist measure uncertainty?
- Consider the resolution of measuring instruments by halving the smallest scale division. 1 degrees Celsius would be 1 plus or minus 0.5.
- Uncertainty = range/2. Greater range = less precise results so greater uncertainty
What are chemical amounts measured in? What is the unit?
Chemical amounts are measured in moles. The symbol for the unit mole is mol.
The mass of one mole of a substance in grams is numerically equal to what?
Its relative formula mass
Eg. 1 mol of Carbon = 12g
MASS IS ALWAYS IN GRAMS SO CONVERT
The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is?
6.02 x 10^23
One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance.
!
What is the equation that links the number of moles, mass and relative formula mass?
Mol = mass/Mr
Define relative formula mass (Mr)
The sum of the relative atomic masses of the atoms in the numbers shown in the formula
Calculate the Mr of CaCO3
Ar(Ca) = 40; Ar(C) = 12; Ar(O) = 16
40 + 12 + (16x3) = 100
Where can the relative atomic mass (Ar) be found?
Mass number of an element on the periodic table
How do you calculate the percentage by mass of an element?
[ Mass of element/Mr of compound ] x 100
Calculate the percentage by mass of beryllium in Be(OH)2
Ar(Be) = 9; Ar(O) = 16; Ar(H) = 1
Mr of compound = 9 + 2(16+1) = 43
Ar of Be = 9
% by mass = 9/43 x 100 = 20.9%
In terms of moles, describe what this chemical equation is displaying:
Mg + 2HCL —> MgCl2 + H2
Ensure that the total number of moles is equal by balancing
1 mole of magnesium is reacting with 2 moles of hydrochloric acid to form 1 mole of magnesium oxide and 1 mole of hydrogen gas
Describe how to use moles to balance chemical equations
- Find the Mr of each reactant and product
- Use this to determine the moles of each reactant and product
- Divide each number of moles by the smallest number of moles. This is the molar ratio for each reactant and product
In the chemical equation below, 1284g of barium chloride reacts with 684g of aluminium sulfate. This forms 1398g of barium sulfate and 534g of aluminium chloride
BaCl2 + Al2(SO4)3 —> BaSO4 + AlCl3
Balance the equation
Mr of BaCl2 = 208.
Mr of Al2(SO4)3 = 342
Mr of BaSO4 = 233
Mr of AlCl3 = 133.5
Mol of BaCl2 = 1248/208 = 6
Mol of Al2(SO4)3 = 684/342 = 2
Mol of BaSO4 = 1398/233 = 6
Mol of AlCl3 = 534/133.5 = 4
Smallest number of moles is 2, so molar ratio is:
6/2 = 3
2/2 = 1
6/2 = 3
4/2 = 2
Therefore, balanced equation:
3BaCl2 + Al2(SO4)3 —> 3BaSO4 + 2AlCl3
Calculate the mass of sodium sulfate that could be formed from 240g of sodium hydroxide. Assume that the sulfuric acid is unlimited
2NaOH + H2SO4 —> Na2SO4 + 2H2O
Number of moles of sodium hydroxide = mass/Mr = 240/40 = 6
6 moles of sodium hydroxide forms 6/2 = 3 moles of sodium sulfate so:
Mass of sodium sulfate = moles x Mr = 3 x 142 = 426 g
What is the limiting reactant?
The reactant that is completely used up first that limits the amount of products
What is the reactant in excess? Why do scientists use an excess of one of the reactants?
The reactant that is left over
To ensure that all of the other reactant is used up
Describe how to calculate the mass of product formed using mass of limiting reactant
- Write out the balanced symbol equation. If it doesn’t say it’s balanced, balance it
- Work out the Mr of the reactant and product you want
- Find out how many moles there are of the substance you want
- Use the molar ratio and then use the number of moles to calculate the mass
What are the two units for concentration?
g/dm^3
mol/dm^3
Always convert if given in cm
Cm^3 —> Dm^3 = divide by 1000
A smaller volume or larger mass affects the concentration how?
Gives a higher concentration
A larger volume or smaller mass affects the concentration how?
Gives smaller concentration
Concentration = mass or mol/?
volume
If the volume of two solutions that react completely are known and the concentration of one solution is not known, the ______ of the other solution can be calculated
Concentration
Equal amounts in ____ of gases occupy the same _____ under the same conditions of _____ and _____
Moles
Volume
Temperature
Pressure
The volume of one mole of any gas at room temperature and pressure is?
24dm^3
What is rtp?
20 degrees Celsius and 1 atmosphere pressure
What is the volume of a gas equal to?
Vol = moles of gas (mass/mr) x molar volume (24dm^3)
This molar volume only applies if the gas is at rtp.
Aluminium oxide can be electrolysed according to the following equation:
2Al203 (s) —> 4Al (s) + 3O2 (s)
(i) Calculate the mass of aluminium that would be produced when 800kg of aluminium is completely electrolysed
(ii) Calculate the volume of oxygen that would be produced when 800kg of aluminium is completely electrolysed
(i) 800kg = 800,000g. mol = mass/mr = 800,000/102 = 7843.137 mol. The molar ratio is 2:4, so moles of Al = 7843.137 x 2 = 15686.274mol
Therefore, mass = mol x Mr = 15686.274 x 27 = 423529g = 423.53kg (1DP)
(ii)
When finding the Mr of halogens, remember they exist as diatomic molecules. Therefore, the Mr of chlorine gas is?
71
What conversions do you need to look out for?
Kg always needs to be in g
Cm^3 always needs to be in dm^3
To get from g/dm^3 to mol/dm^3, you multiply by the Mr. Always check if they want your answer in this form
Find the hard quantitative question
Calculate the number of MOLES of ATOMS in one mole of calcium hydroxide, Ca(OH)2
1 molecule of calcium hydroxide contains 5 atoms
1 mole of calcium hydroxide molecules contains 5 moles of atoms
Calculate the NUMBER OF ATOMS in one mole of hydrogen chloride, HCl
1 mole of HCl contains Avogadro’s constant of molecules
As HCl has 2 atoms, the number of atoms = 2 x 6.02 x 10^23
= 1.204 x 10^24 atoms
Calculate the number of atoms in 54g of water. Ar(H) = 1 and the Ar(O) =16
Moles of water = 54/18 = 3mol
The number of molecules in one mole is avogadro’s constant.
Number of molecules in 3 moles = 3 x 6.02 x 10^23 = 1.806 x 10^24
Therefore, the number of atoms = 3 x 1.806 x 10^24 = 5.418 x 10^24
If there are fractions in chemical equations, what do I do?
Multiply to get whole numbers.
If you have 4.8 g of magnesium ribbon reacting in a solution of dilute, hydrochloric acid containing 7.3 g of hydrochloric acid, which reactant is the limiting reactant?
1) Balanced symbol equation is Mg + 2HCl —> MgCl2 + H2
2) Mol of Mg = 0.2
3) Mol of HCl = 0.2
As the molar ratio is 1:2, 0.2 mol of Mg will need 0.4 mol of HCl to react completely. As we only have 0.2mol, dilute hydrochloric acid is the limiting reagent (and magnesium is in excess)
What is yield?
The mass of a product a chemical reaction produces
What is the equation for percentage yield?
Percentage yield = actual mass of product produced/maximum theoretical mass of product possible x 100
What four factors may affect percentage yield?
The reaction may not go to completion because the reaction may be reversible
Some of the desired product may be lost when it is separated
The reactants may not be as pure
Some may react to give unexpected or waste products
Define atom economy
The atom economy or atom utilisation is a measure of the amount of starting materials that end up as useful products
Why is atom economy important?
For sustainable develop,ent
For economic reasons to use reactions with high atom economy
Students should be able to explain why a particular reaction pathway is chosen to produce a specified product given appropriate data such as atom economy, if not calculated, yield, equilibrium position, and usefulness of byproducts
How can the concentration of an aqueous solution be increased?
Add more solute and dissolving it in the same volume of its solution
Evaporating off some of the water so you have the same mass of solute in a smaller volume of solution
What are concordant results?
Use the practical handbook for practicals
Number of particles = ?
Number of moles x Avogadro’s constant
Atom economy formula
Atom economy = mr desired product / mr of all reactants x 100
