1.3 Chemical calculations

Question 1

What is relative atomic mass?

Answer 1

The mass of an atom compared to 1/12 of the mass of a carbon-12 atom

Question 2

What is relative molecular mass?

Answer 2

The mass of a molecule compared to 1/12 of the mass of a carbon-12 atom

Question 3

What is relative isotopic mass?

Answer 3

The mass of an isotope compared to 1/12 of the mass of a carbon-12 atom

Question 4

How to determine the group and period of an element?

Answer 4

Use electron arrangement

E.g sodium 2,8,1

3 shells means it’s in PERIOD 3
1 electron in the last shell means it’s in GROUP 1

Question 5

How to determine the charges of an element?

Answer 5

If an element ion is in group 1-4, the charge should be +1 to +4

If an element ion is in group 5, the charge should be -3

If an element ion is in group 6, the charge should be -2

If an element ion is in group 7, the charge should be -1

If an element ion is in group 8, the charge should be 0

Question 6

What is an element?

Answer 6

an element only consists of one type of atom

Question 7

What is relative formula mass?

Answer 7

sum of relative atomic mass (Mr) in a compound. Same as relative molecular mass

Question 8

Define mole

Answer 8

The number of carbon 12 atoms with a mass of 12.0 grams

Avagadro’s number ➡️ 6.02 x 10²³ particles, (which may be atoms, molecules, ions, or electrons.)

Question 9

Define molar mass

Answer 9

The mass in grams of 1 mole of a substance (same VALUE as molecular mass which = adds up all the relative atomic mass)

e.g the molar mass of H₂ is 1.01 x 2 = 2.02

Question 10

How to work out the number of moles?

Answer 10

moles(mol) = mass(g) ÷ molar mass(mr)

Question 11

How to work out the mass?

Answer 11

mass(g) = moles(mol) x molar mass(mr)

Question 12

How to work out the molar mass?

Answer 12

molar mass(mr) = mass(g) ÷ moles(mol)

Question 13

Find the number of moles in 9.01 grams of water

Answer 13

moles = mass/mr water = H₂O
= 9.01/(1.01x2+16.0)
=9.01/18.02
=0.5

Question 14

Q1. Find out the mass of iron produced when 11.12 grams of iron(iii) oxide is reduced with carbon

Answer 14

Step 1: find out the number of moles of the thing you know (iron(iii) oxide) = Fe₂O₃
moles of iron(iii) oxide
= mass/mr = 11.12/(55.8x2 + 16.0x3)
= 11.12/159.6 = 0.0697(LEAVE THE ANSWER IN THE CALCULATOR)

Step 2: Use the equation(if not given, write down a BALANCED equation yourself) to find the moles of iron produced
2Fe₂O₃ + 3C ➡️ 4Fe + 3CO₂

The number of an element/molecule is the NUMBER in front of them!!! e.g the moles of Fe₂O₃ is 2, the moles of Fe is 4.

Therefore 2 moles of Fe₂O₃ ≡ 4 moles of Fe
so 1 mole of Fe₂O₃ ≡ 2 moles of Fe
(≡ means identical to)

So 0.0697 mole of Fe₂O₃ ≡ 0.0697x2 = 0.139…(leave ans in cal)

Step 3: Calculate the mass of Fe using mass = moles x mr
mass of Fe = 0.139 x 55.8 = 7.78 g (3.s.f)

Always leave the unit in 3 significant figure!!!

Question 15

What is empirical formula?

Answer 15

The simplest whole number ratio of atoms in a molecule

e.g Alkenes has the simplest formula CH₂

But the chemical formula is different, 
ethene: C2H4
propene: C3H8
Butene: C4H8
Pentene: C5H10

Question 16

How to find out the chemical formula of a compound?

Answer 16

Empirical formula and mr

Question 17

What’s the rule of empirical formula?

Answer 17

Always has to be intergers!!1

e.g if 1 : 1.5, it has to multiply 2 to become 2 : 3
if 1 : 1.3 recurring, multiply by 3 to become 3 : 4
if 1 : 1.25, multiply by 4 to become 4 : 5

Question 18

Q1. find the empirical formula of a compound that contains 27.3 grams of carbon and 72.7 grams of oxygen

Answer 18

Step 1 : write a mass ratio
C : O
27.3 : 72.7

Step 2 : divide each by their relative atomic mass
C : 27.3/12.0 = 2.275
O : 72.7/16.0 = 4.55

Step 3: divide by the smallest (in step two)
Number of C = 2.275/2.275 = 1
Number of O = 4.55/2.275 = 2

So the empirical formula is CO₂

Question 19

What is 1dm³ in cm³?

Answer 19

1000cm³

Question 20

What is STP?

Answer 20

Standard temperature and pressure
273K (0°C) and 1 atm (atmosphere)
molar volume at 22.4 dm³

Question 21

What is RTP?

Answer 21

Room temperature and pressure
293K (20°C and 1 atm (atmosphere)
molar volume at 24.0 dm³

Question 22

How to work out the volume of a gas?

Answer 22

volume(dm³) = moles(mol) x molar volume(dm³)

Question 23

How to work out the moles of a gas?

Answer 23

moles(mol) = volume(dm³)/molar volume(dm³)

Question 24

Q1. Calculate the volume of carbon dioxide evolved at r.t.p when an excess of dilute hydrochloric acid is added to 1g of calcium carbonate (where Vₘ = 24dm³)

Answer 24

CaCO₃ + 2HCl ➡️ CaCl₂ + CO₂ + H₂O

1 mole of CaCO₃ = 1 mole of CO₂
mole of CaCO₃ = mass/mr = 1/100.1

1/100.1 mole of CaCO₃ ≡ 1/100.1 mole of CO₂
volume of CO₂ = moles x molar volume
= 1/100.1 x 24
= 0.240dm³ (3.s.f)

Question 25

What is the ideal gas law?

Answer 25

At Standard Temperature and Pressure (STP), 1 mole of any gas will occupy a volume of 22.4dm³

Question 26

What are concentrations measured in?

Answer 26

mol dm⁻³ means 1 mole per dm³

Question 27

How to find the moles in terms of concentration?

Answer 27

moles = (concentration x volume(cm³) ÷ 1000

Question 28

How to find the concentration?

Answer 28

concentration = (moles x 1000) ÷ volume(cm³)

Question 29

How to find the volume in terms of concentration?

Answer 29

volume(cm³) = (moles x 1000) ÷ concentration

Question 30

Q1. 0.1 mol dm⁻³ of HCl has an average titre of 23.20 cm³ mixed with 25 cm³ of NaOH. What's the concentration of NaOH?

Answer 30

moles of HCl = conc x volume / 1000 = 0.1 x 23.20 / 1000 = 2.32 x 10⁻³ moles HCl + NaOH ➡️ NaCl + H₂O 1 mole of HCl ≡ 1 mole of NaOH 2.32 x 10⁻³ moles of HCl ≡ 2.32x10⁻³ moles of NaOH Conc of NaOH = moles x 1000 / volume = 2.32x10⁻³ x 1000 / 25 =0.0928 mol dm⁻³