Relative/ Formula mass and Equations

Question 1

How do you calculate formula mass (Mr/RFM)?

Answer 1

1. Find atomic mass
2. Times by small number
3. Add

E.g SiO2

18 + 32 = 50

Question 2

How to calculate percent composition?

Answer 2

Total mass of element in compound/ x 1000

RFM of compound

Question 3

Avagadro’s number

Answer 3

6.03 x 10 ₂₃

How many particles are there in 1 mole of cake?
6.03 x 10 sub>23

Question 4

Mole

Answer 4

Number of a specific quantity of particles

Question 5

What’s the equation relating mass, moles and RFM?

Answer 5

Mass = moles x RFM

Question 6

Calculate mass of oxygen needed to react to 10g of calcium to form calcium oxide

2Ca + O2 → 2CaO

Answer 6

Mass: 10g 4g

RFM: 40 32 (atomic number x small number)

mol: 0.25 0.125

Ratio: 2 1 (big number)

Question 7

What does the molecular formula tell us?

Answer 7

The exact number of atoms in a molecule

Question 8

What does the empirical formula tell us?

Answer 8

The simplest ratio of atoms in a molecule

Question 9

How do we calculate the empirical formula?

Answer 9

1. Divide mass/ percentage of element by RFM
2. Calculate ratio by dividing by the smallest value

E.g
Al : 52.9% / 27 O: 47.1% / 16
1 : 1.5
2 : 3

Simple whole ratio:

• 1.33 : 1 → 4 : 3
• 1.68 : 1 → 5 : 3
• 1.24 : 1 → 7 : 4

Question 10

How to calculate percentage yield?

Answer 10

Percentage yield = actual yield / theoretical yield x 100

Question 11

Why might percentage yield not be 100 % ?

Answer 11

• loss of reactants
• side reactions occurring
• reversible reaction

Question 12

What does 1 mole of gas occupy at rtp?

Answer 12

24 dm 3
or
24,000 cm3

Question 13

What’s the equation relating volume, moles and 24dm3

Answer 13

Volume = moles x 24dm3

Question 14

Calculate volume of 0.01 g of Hydrogen gas at rtp

Answer 14

Mol = mass/RFM

Mol = 0.01/2

= 1/200

1/200 x 24,000 = 120 cm3

Question 15

RAM

Answer 15

Average mass of isotopes of an element compared to the mass of a Carbon 12 atom

Question 16

How do you calculate RAM?

Answer 16

1. Multiply each isotope by abundance
2. Add total together
   3.Divide by total abundance number
   (may not be a hundred )

Question 17

Isotopes

Answer 17

Atoms of the same element, with the same number of protons but different numbers of neutrons

Question 18

The empirical formula of sodium chloride is NaCl. It does not have a molecular formula. Why is this the case?

Answer 18

Sodium chloride doesn’t exist as a molecule. It’s a giant ionic lattice

Question 19

Theoretical yield

Answer 19

The quantity of products that would be obtained from a complete reaction

Question 20

Actual yield

Answer 20

The amount of products actually obtained from a reaction

Question 21

What is the mass of one mole of a substance?

Answer 21

Its RFM, measured in grams

Question 22

How would you calculate percentage composition of A in compound B?

Answer 22

% A = (RAM * no.of atoms of A in B) / RFM of B * 100