Quantitative - Amounts Of Substances

Question 1

What is relative formula mass (Mr)

Answer 1

All the relative atomic masses in a compound added together

Question 2

What is percentage mass

Answer 2

The percentage of an element within a compound

E.g. the percentage mass of carbon in methane is 75%. So methane is made of 75% carbon atoms

Question 3

How do you work out the percentage mass of an element in a compound

Answer 3

Relative atomic mass of element x nummber of atoms of that element

÷ relative formula mass of compound

Then multiply by 100

E.g. percentage mass of magnesium in magnesium oxide is:

24 × 1 ÷ 40 ×100 = 60%

Question 4

A mixture contains 20% iron ions by mass. The only compound in the mixture that contains iron is iron chloride
(FeCl2). Calculatr the mass of iron chloride in 50g of the substance

Fe= 56
Cl= 35.5

Answer 4

Find the mass of iron in the mixture
20/100 x 50 = 10g iron

Work out relative formula mass of iron chloride
56 + (2 x 35.5) = 127

So percentage mass of iron in iron chloride is
56/127 x 100 = 44.09%

Mass of iron chloride x 44.09/100 = 10g

Mass of iron chloride = 23g

Question 5

What is the avogardro constant

Answer 5

6.02 x 10^23

Question 6

What is one mole

Answer 6

The amount of substance that contains 6.02 x 10^23 particles

Question 7

The relative atomic mass of iron is 56

How much does one mole of iron weigh

Answer 7

56g

Question 8

What is the formula you can use to work out nummber of moles

Answer 8

Mass (g) / relative atomic (or formula) mass

Question 9

What is the connection between the relative formula mass of the reactant and the products in a reaction

Answer 9

It’s the same

Question 10

Mg + 2HCL = MgCl2 + H2

How many moles of each substance are there in this equation

Answer 10

Mg = 1
HCL = 2
MgCl = 1 
H2 = 1

Question 11

How many moles of oxygen will react if 3 moles of magnesium react completely fo form magnesium oxide

2Mg + O2 = 2mgO

Answer 11

1.5 moles of oxygen

Question 12

8.1g of zinc oxide (ZnO) reacts completely with o.6g of carbon to form 2.2g of carbon dioxide and 6 5g of zinc.

Write a balanced symbol equation for this reaction

Ar: c=12 O=16 Zn= 65

Answer 12

Work out ar for each substance in the reaction
ZnO = 81 c = 12 CO2 = 44 Zn = 65

Work out how many moles of each substance there where
Moles = mass/mr
Zno is 0.1 co2 is 0.05 c is 0.05 Zn is 0.1

Divide by the smallest nummber of moles u just got ( 0.05)
If the numbers u then get are whole numbers, put them in your equation

2ZnO + C = CO2 + 2Zn

Question 13

What mass of calcium chloride (CaCl2) is produced when 3.7g of calcium hydroxide (Ca(OH)2) reacets with an excess of hydrochloric acid

Ca(OH)2 + 2hcl = CaCl2 + 2H2O

Answer 13

Mr of caOH2 is 74
Mr if calcl2 is 111

Find moles of calcium hydroxide 3.7÷74 = 0.05 moles

So there will also be 0.05 moles of calcium chloride because they have a 1:1 ratio in the balanced symbol equation

Moles × mr = mass

0.05 × 111 = 5.6g

Question 14

At room temp and pressure, one mole of any gas occupies what space

Answer 14

24dm^3

Question 15

What is the equation for the volume of gas (dm3)

Answer 15

Mass/Mr × 24

Question 16

How much carbon dioxide is formed when 30dm of oxygen reacts with carbon monoxide

2CO + O2 = 2CO2

Answer 16

The ratio of oxygen to carbon dioxide is 1:2 so it will be 60