Chemistry 1

Question 1

True or false? The Mr of a compound is always greater than the Ar of any of the elements in that compound.

Answer 1

True - you find the Mr by adding up the Ar of each element, so it must be greater.

Question 2

The Ar of oxygen is 16 and the Ar of nitrogen is 14.

Find the Mr of nitric oxide, NO.

Answer 2

14 + 16 = 30.

Question 3

How do you calculate the percentage mass of an element in a compound?

Answer 3

Multiply the Ar of the element by the number of atoms of that element in the formula of the compound. Then divide by the compound’s Mr, and multiply by 100.

Question 4

The formula of the compound sulfur hexafluoride is SF6.

Calculate the Mr of sulfur hexafluoride.

Ar of F = 19 and Ar of S = 32.

Answer 4

32 + (19 x 6) = 32 + 114 = 146.

Question 5

The Mr of methane, CH4, is 16. The Ar of carbon is 12.

What is the percentage mass of carbon in methane?

Answer 5

(12 ÷ 16) x 100 = 0.75 x 100 = 75%.

Question 6

Find the Mr of calcium hydroxide, Ca(OH)2.

Ar of Ca = 40, Ar of O = 16, Ar of H = 1.

Answer 6

40 + [2 x (16 + 1)] = 40 + [2 x 17] = 40 + 32 = 74.

The little two is outside the brackets in this formula, so remember, that means there’s two of everything inside the brackets.

Question 7

Compound X has Mr = 30 and contains 10% hydrogen by mass. Ar of H = 1.

How many hydrogen atoms are there in the molecular formula of compound X?

Answer 7

Mass of H = 10% of Mr of X = 10% of 30 = 3.

So number of H atoms = 3 ÷ (Ar of H) = 3 ÷ 1 = 3.

Question 8

What is the definition of a mole? (The chemistry thing, not the animal.)

Answer 8

One mole is 6.02 x 10^23 (the Avogadro constant) particles of a substance.

It is the number of atoms of Carbon-12 in 12g of Carbon-12.

Question 9

What does the Ar or Mr of a substance tell you about the mass of one mole of that substance?

Answer 9

The value of Ar of an element or the Mr of a compound is equal to the mass in grams of 1 mole of the substance.

E.g. the Mr of CO2 is 44, so 1 mole of CO2 has a mass of 44g.

Question 10

Which contains more particles, a mole of water or a mole of oxygen gas?

Answer 10

Neither - they both contain the same number of particles.

1 mole of water contains more atoms, but there are 6.02 x 10^23 molecules of H2O or O2 in 1 mole.

Question 11

What’s the formula for the number of moles in a given mass?

Answer 11

Number of moles = mass in grams ÷ Mr.

Question 12

The Mr of sodium hydroxide, NaOH, is 40. How many moles are there in 500g of NaOH?

Answer 12

Number of moles = mass ÷ M r = 500 ÷ 40 = 12.5 moles.

Question 13

3 moles of compound A have a mass of 126g. What is the Mr of compound A?

Answer 13

Mr = mass ÷ number of moles = 126 ÷ 3 = 42.

Question 14

What’s the mass of 4 moles of potassium fluoride, KF? Ar of K = 39, Ar of F = 19.

Answer 14

Mass = number of moles x Mr = 4 x (39 + 19) = 4 x 58 = 232g.

Question 15

The formula of magnesium bromide is MgBr2. Ar of Mg = 24, Ar of Br = 80.

What mass of bromine is there in 0.25 moles of MgBr2?

Answer 15

1 mole of MgBr2 contains 2 moles of Br atoms, so 0.25 moles contains 0.5 moles of Br. So the mass of Br in 0.5 moles is 0.5 x 80 = 40g.

Question 16

What is meant by the term ‘conservation of mass’?

Answer 16

Total mass of products = Total mass of reactants.

This is because no atoms are ever destroyed or created during a reaction.

Question 17

How does the balanced symbol equation for a reaction show that mass is conserved in that reaction?

Answer 17

There will be the same number of each type of atom on both sides of the equation.

Question 18

Over a period of two weeks, the mass of a lump of metal left in an open container has increased. Suggest why.

Answer 18

The metal will have reacted with one or more of the gases in the air, such as oxygen. The new compound will have a greater mass than the metal alone.

Question 19

The equation for the reaction between sulfuric acid and sodium carbonate is:

H2SO4(aq) + Na2SO3(aq) → Na2SO4(aq) + H2O(l) + CO2(g)

This reaction was carried out in an open beaker. Predict whether the mass of the beaker and its contents will increase, decrease or stay the same during the course of the reaction. Explain your answer.

Answer 19

The mass will decrease because one of the products is carbon dioxide gas, which will escape the open beaker.

Question 20

The reaction between sodium and water is:

2Na + 2H2O → 2NaOH + H2

What is the ratio of the total number of moles of reactants to the total number of moles of products in this reaction?

Answer 20

4:3

Every 2 + 2 = 4 moles of products makes 2 + 1 = 3 moles of products.

Question 21

Zinc reacts with oxygen to form zinc oxide, ZnO, as follows:

2Zn + O2 → 2ZnO

Find the total number of moles of reactants needed to make 8 moles of ZnO.

Answer 21

The equation makes 2 moles of ZnO, so to make 8 moles of ZnO, multiply by 8 ÷ 2 = 4:

8Zn + 4O2 → 8ZnO

So making 8 moles of ZnO uses 8 + 4 = 12 moles of reactants.

Question 22

0.9 moles of compound A react completely with 0.6 moles of compound B to form 0.6 moles of compound C, which is the only product of this reaction. Find the balanced equation for this reaction in terms of A, B and C.

Answer 22

Divide all the numbers of moles by the smallest number of moles (0.6):

A: 0.9 ÷ 0.6 = 1.5
B: 0.6 ÷ 0.6 = 1
C: 0.6 ÷ 0.6 = 1

Now multiply to get them all to be whole numbers. So in this case, multiply by 2:

A: 1.5 x 2 = 3
B: 1 x 2 = 2
C: 1 x 2 = 2.

So the balanced equation is: 3A + 2B → 2C

Question 23

Compound Z decomposes to make nitrogen gas, N2, and sodium, Na. When 2 moles of Z decomposes, it produces 46g of sodium and 84g of nitrogen.

Deduce the balanced equation for the decomposition of Z, in terms of Z, N2 and Na. Ar of Na = 23, Ar of N = 14.

Answer 23

46g of Na = 46 ÷ 23 = 2 moles.

84g of N2 = 84 ÷ (2 x 14) = 84 ÷ 28 = 3 moles.

The numbers of moles are all whole numbers, so the balanced equation is:
2Z → 2Na + 3N2.

Question 24

What’s the definition of a limiting reactant?

Answer 24

A limiting reactant is the reactant that gets completely used up in a given reaction.

Question 25

What does it mean if a reactant is said to be in excess?

Answer 25

A reactant is in excess if there is still some left after the reaction has finished.

Question 26

Magnesium reacts with hydrochloric acid as follows:

Mg + 2HCI → MgCI2 + H2

How many moles of magnesium chloride, MgCI2, form when 0.4 moles of magnesium react with an excess of hydrochloric acid?

Answer 26

0.4 moles.

The equation shows you get 1 moles of MgCI2 for each moles of Mg that reacts. The acid is in excess, so all 0.4 moles of magnesium will react to make 0.4 moles of MgCI2.

Question 27

Mr Mole lives under a…

Answer 27

… Massive Mound

This is a way of remembering the calculation triangle for moles. Mass is on top and Mr (Relative Molecular Mass) and Mole are at the bottom.

Question 28

Matthew’s favourite drink is…?

Answer 28

Green Tea