Quantitative Chemistry

Question 1

Mass number definition

Answer 1

The number of protons and neutrons in the nucleus of an atom
So if in an atom has 4 protons and 5 neutrons, it’s mass number is 4 + 5 = 9

Question 2

How do you find the relative formula (molecular) mass (Mr)

Answer 2

Add together the mass numbers of the molecule (Mr)

Eg
H2SO4 = (2x1) + 32 + (16+4) = 98

Question 3

What is the atomic number

Answer 3

The number of protons

Mostly also the number of electrons as the charges balance each other out

Question 4

How do you calculate the % of an element in a compound

Answer 4

Mass of element/ Mr of compound x 100

Question 5

Relative formula mass (I made it up)

Answer 5

The mass of the molecule

Question 6

Relative atomic mass (I made it up)

Answer 6

The mass of the atom/ element

Question 7

What is a mole

Answer 7

One mole of a substance always contains the same number of particles

Question 8

Equation to find moles

Answer 8

Number of moles =
Mass/ Ar or Mr

Eg
Number of moles for 23g sodium = 23g/ 23 = 1 mole

Question 9

What is empirical formula

Answer 9

The simplest whole number ratio of atoms of each element in one molecule of a compound

Question 10

How do we find the empirical formula

Answer 10

Find the number of moles of each element in the molecule
Divide by the smallest

Eg
If 9g of Al react with 35.5g of Cl, what is the empirical formula of the compound formed

Al: 9/27 = 0.33333 moles
Cl: 35.5/35.5 = 1 mole

0.33333/0.33333 = 1
1/0.33333 = 3 

AlCl3

Question 11

How do we find the molecular formula when given the empirical formula

Answer 11

Find Mr of empirical
Mr of the compound/ Mr of the empirical
Times the empirical formula by this number

Eg
The empirical formula of a compound is found to be AlCl3. The Mr of the compound is 267 so what is the molecular formula

Mr of empirical = 27 + (35.5x3) = 133.5

267/ 133.5 = 2

Al(1x2)Cl(3x2) = Al2Cl6

Question 12

How do you calculate water of crystallisation

Answer 12

Find the mass of compound and water
Find the moles of the compound and water
Divide by the smallest

Eg
1.23g hydrated magnesium sulphate was heated until constant mass was achieved and there was mass loss of 0.63g. Calculate the formula of the hydrated crystals

Mass of MgSO4 = 1.23-0.63 = 0.6g

MgSO4: 0.6/120 = 0.005 moles
H2O: 0.63/18 = 0.035

0. 005/0.005 = 1
1. 035/0.005 = 7

MgSO4•7H2O

Question 13

How to find the mass of a product/reactant when given the moles of one the reactants/products

Answer 13

Calculate the number of MOLES of the substance that you have the mass of
Find the number of MOLES of the substance you want to find the mass of from the ratios of the balanced equation
Calculate the MASS by multiplying the moles in step 2 by Mr

Eg
10g of Mg in the air produces MgO, what is the mass of MgO produced?
2️⃣Mg + O2 —> 2️⃣MgO

Moles of Mg = 10/24 = 0.416
Moles of MgO = 0.416 (1:1 ratio)
Mass of MgO = 0.416 x 40 = 16.7g (3sf)