4.5.2 Chemical Quantities

Question 1

How many elements are there in the periodic table?

Answer 1

Over 100

Question 2

What atoms are present in the compound H20.

Answer 2

This has 2 Hydrogen atoms and 1 oxygen atom

Question 3

What atoms are present in the compound HNO3

Answer 3

This has 1 hydrogen atom, 1 nitrogen atom and 3 oxygen atoms

Question 4

What atoms are present in the compound Mg(OH)2

Answer 4

This has 1 magnesium atom, 2 oxygen atoms and 2 hydrogen atoms as the two outside the bracket means you multiply everything inside the bracket by 2.

Question 5

What do the symbols (s), (l) and (g) mean when they are written in an equation?

Answer 5

s  = solid
l = liquid
g  = gas

Question 6

What does the symbol (aq) mean when it is written in an equation?

Answer 6

Aqueous - this means dissolved in water.

Question 7

What is the conservation of mass law?

Answer 7

The law of conservation of mass states that
no atoms are lost or made during a chemical
reaction so the mass of the products equals the
mass of the reactants

Question 8

H2+ Br2 -> HBr

Has the conservation of mass law been applied in this equation?

Answer 8

No, on the left there are 2 hydrogen atoms and 2 bromine atoms reacting. On the right there is only one hydrogen atom and one bromine atom - it looks like mass has disappeared.

Question 9

What do we mean by reactant?

Answer 9

A substance which is reacting together with something else in a chemical reaction. They are found on the left of the arrow in an equation.

Question 10

What do we mean by a product?

Answer 10

A substance which is produced or made in a chemical reaction. They are found on the right of the arrow in an equation.

Question 11

Copper carbonate –> Copper oxide + carbon dioxide

In this reaction the mass of the test tube appears to go down. The conservation of mass law says this is not possible. Where has the mass gone?

Answer 11

Carbon dioxide is a gas so has left the test tube.

Question 12

1g Magnesium is heated in the air. Afterwards it is weighed and has a mass of 1.2g. The conservation of mass law says that mass cannot just appear. Where has this mass come from?

Answer 12

The magnesium has reacted with oxygen atoms from the air

Magnesium + Oxygen –> Magnesium Oxide

Question 13

Which element in the periodic table do we compare all other elements to when we find their mass?

Answer 13

Carbon 12

Question 14

Use your periodic table to answer this question.

What is the relative formula mass of H20?

Answer 14

H2o = H + H + O

= 1 + 1 + 16 = 18

Question 15

Use your periodic table to answer this question.

What is the relative formula mass of CaCO3?

Answer 15

CaCO3 = Ca + C + O + O + O

= 40 + 12 + 16 + 16 +16 = 100

Question 16

Use your periodic table to answer this question.

What is the relative formula mass of Al(OH)3

Answer 16

Al(OH)3 = Al + O + O + O + H + H + H

= 27 + 16 + 16 + 16 + 1 + 1 + 1 = 78

Question 17

Hydrogen + Oxygen -> Water

If I have 12.3g of Hydrogen and 8g of oxygen - how much water must I make?

Answer 17

20.3g as all mass is conserved

Question 18

Hydrogen + Nitrogen –> Ammonia

If I start with 8g of Hydrogen and make 16.5g of ammonia - how much nitrogen must I have reacted with?

Answer 18

8.5g as the mass of all the reactants has to equal the mass of the products

Question 19

What is a mole?

Answer 19

It is a number we use to count the number of atoms present. 1 mole = 6.02 x10power23 atoms! or 602000000000000000000000 atoms!

This is called avagadros constant

Question 20

What is the equation to calculate moles?

Answer 20

Moles = mass (in grams) divided by relative formula mass

Question 21

How many moles are there in 9g of water H20?

Answer 21

moles = mass divided by relative formula mass

relative formula mass for water = 18 (using periodic table)

moles = 9 divided by 18 = 0.5 mol

Question 22

What do we mean by limiting reagent?

Answer 22

It is the reacting chemical which is completely used up in the reaction

Question 23

What do we mean by ‘in excess’?

Answer 23

There will be some of the reactant left over, not all of it will react in the reaction.

Question 24

What is the equation for concentration of a solution?

Answer 24

Concentration = mass divided by volume (in dm3)

Question 25

How do you convert cm3 into dm3?

Answer 25

1dm3 contains 1000cm3 of liquid.

25cm3 therefore is 0.025dm3

Question 26

What is the concentration of a solution containing 12g of salt dissolved in 500cm3 of water?

Answer 26

Concentration = mass divided by volume (in dm3)

500cm3 = 0.5dm3

Concentration = 12/0.5 = 24 g/dm3

Question 27

What is the name of a compound made of magnesium and oxygen?

Answer 27

Magnesium Oxide - you always list the metal first and then the non metal. The non metals name is changed to end in ‘-ide’.

Question 28

What is the name of the compound made of bromine and sodium?

Answer 28

Sodium Bromide - you always list the metal first and then the non metal. The non metals name is changed to end in ‘-ide’.

Question 29

What is the name of the compound made of copper sulphur and oxygen?

Answer 29

Copper Sulphate - you always list the metal first and then the non metal. If oxygen is also present the non metal name is changed to end in ‘-ate’.

Question 30

What is the name of the compound made of nitrogen, oxygen and iron.

Answer 30

Iron Nitrate - you always list the metal first and then the non metal. If oxygen is also present the non metal name is changed to end in ‘-ate’.