P1P2C9 - Calcs involving masses

Question 1

What is Ar?

Answer 1

Relative atomic mass

Question 2

What is Mr?

Answer 2

Relative molecular/formula mass

Question 3

How do we find Mr?

Answer 3

Add the Ar’s

Question 4

Calculate the Mr of Na2O
Na = 23
O = 16

Answer 4

(23x2) + 16
= 62
Mr = 62

Question 5

What is empirical formula?

Answer 5

The simplest ratio of ions which make up a molecule.

Question 6

What is a molecular formula?

Answer 6

The actual amount of atoms which make up a molecule.

Question 7

What’s the experiment for calculating empirical formula?

Answer 7

1. Measure mass of solid that’s going to be reacted (e.g. magnesium ribbon).
2. React solid with oxygen in air by heating.
3. Find out mass of product (magnesium oxide).
4. Calculate mass of oxygen gained (final mass - initial mass).
5. Use mass of magnesium and oxygen to calculate the empirical formula.

Question 8

How do you calculate empirical formula?

Answer 8

Look at masses of the element in Q
Divide by Ar
Divide by smallest
Get formula

Question 9

A solution contains 21.6 of carbon, 6.3 of hydrogen and 14.4 of oxygen.
Whats the empirical formula?
Carbon = 12
Hydrogen = 1
Oxygen = 16

Answer 9

(21.6/12) = 1.8
(6.3/1) = 6.3
(14.4/16) = 0.9

1.8/0.9 = 2 Carbons
6.2/0.9 = 7 Hydrogens
0.9/0.9 = 1 Oxygen

= C2H7O

Question 10

What is the law of conservation of mass?

Answer 10

Total mass of reactants = Total mass of products

Question 11

If the total mass of reactants in a chemical reaction is 20g, what can we say about the total mass of products?

Answer 11

Equal 20g

Question 12

What units can concentration be measured in?

Answer 12

g / dm3

mol / dm3

Question 13

How do you calculate concentration?

Answer 13

Concentration =
Mass / Volume

Question 14

What are some examples of reactions where mass changes?

Answer 14

Wood burning
Corrosion
Zinc + hydrochloric acid

Question 15

What is a mole?

Answer 15

A measurement of chemical amounts

Question 16

What is Avogadros constant?

Answer 16

6.02 X 1023

Question 17

How do you calculate moles?

Answer 17

Moles =
Mass / Ar (Relative atomic mass)

Question 18

2Na + H2SO4 → Na2SO4 + H2

Calculate the mass of each of the substances given that there is 0.1 moles of sodium.

Ar (Na) = 23
Mr (H2SO4) = 98
Mr (Na2SO4) = 14
Mr (H2) = 2

Answer 18

1. Work out how many moles of each substance is involved in the reaction.
   H2SO4 = 0.1 / 2 = 0.05mol.
   Na2SO4 = 0.1 / 2 = 0.05mol.
   H2 = 0.1 / 2 = 0.05mol.
2. Recall the equation for calculating mass.
   Mass = moles x Mr
3. Calculate the mass of each substance involved.
   Na = 0.1 x 23 = 2.3g.
   H2SO4 = 0.05 x 98 = 4.9g.
   Na2SO4 = 0.05 x 142 = 7.1g.
   H2 = 0.05 x 2 = 0.1g.

Question 19

What is a limiting reactant?

Answer 19

The reactant that is completely used up.
Because the reaction stops when it is used up.