1 Chemical formulae, equations and calculations

Question 1

What is very important to remember when balancing an equation?

Answer 1

There should be the same amount of the atom on the right side and left side of the equation.

Question 2

What are the steps that you should follow when balancing an equation?

Answer 2

• Work across the equation from left to right, and check one element at a time (one after the other).
• Check everything at the end to make sure that you haven’t changed something that you haven’t counted.

Question 3

What are the different state symbols?

Answer 3

• (s)
• (l)
• (g)
• (aq)

Question 4

What is the relative formula mass?

Answer 4

It is the sum of the relative atomic masses of the atoms in the number shown in the formula

Question 5

What is something very important to remember when calculating the relative formula mass?

Answer 5

That the big number in front of an element isn’t counted in order to calculate it.

Question 6

What is a mol?

Answer 6

A unit of the amount of substance.

Question 7

What is the equation to find out the number of moles?

Answer 7

N = m (mass g) /mr

Question 8

What unit does m (mass) always need to have?

Answer 8

grams

Question 9

How do you calculate the % of something in a compound?

Answer 9

• Write down the formula of the compound.
• Find out its molecular mass.
• Write down the mass of the element as a fraction of the total.
• Multiply by 100 to find the %.

Question 10

How do you calculate reacting mass step by step?

Answer 10

1) Check the ratio and write it down
2) Find the number of moles for the compound that you have the mass of.
3) Use the ratio method to find out the number of moles for the other compound.
4) After that, on the other compound, you find out its mass by using the value of x that you found in the ratio method.

Question 11

Why may you not get 100% yield?

Answer 11

• Some of the product may be lost when it is separated from the reaction mixture.
• Reversible reactions may not go to completion.

Question 12

How do you calculate percentage yield?

Answer 12

Mass of product actually made / x100
Maximum theoretical mass of product

Question 13

How do you know if your percentage yield calculation is wrong?

Answer 13

It is above 100%, which would mean that we would have created atoms.

Question 14

What is the method used to investigate the formula of a metal oxide by combustion?

Answer 14

• Weigh a crucible with its lid.
• Place a piece of magnesium in this and reweigh.
• Heat the crucible by using a roaring flame.
• Lift the lid every few seconds.
• When the reaction is finished, allow the crucible and contents to cool.
• Weigh everything altogether.

Question 15

What is produced in this reaction?

Answer 15

Magnesium oxide - a white powder.

Question 16

How do you work out the mass of magnesium?

Answer 16

M of crucible - m of crucible with magnesium.

Question 17

How do you work out the mass of oxygen?

Answer 17

M of crucible at the end - m of crucible with magnesium.

Question 18

How do you find the formula of magnesium oxide?

Answer 18

By using the empirical formula to find Mg and O.

Question 19

What is the method used to investigate the formula of a metal oxide by reduction?

Answer 19

• Weigh a ceramic dish.
• Put about 3g of copper oxide in the ceramic dish and reweigh.
• Place the ceramic dish in a tube.
• Pass hydrogen gas over the copper oxide.
  -Ignite the excess hydrogen, which comes out of the small hole in the boiling tube.
• Heat the copper oxide strongly until the reaction is finished.

Question 20

How will you know that the reaction is finished?

Answer 20

A pink-brown copper metal will be seen.

Question 21

How do you determine the formula of water?

Answer 21

You use the same apparatus and method but you condense the water vapour at the end.

Question 22

What do you do if you are given an x in a formula?

Answer 22

You find the empirical formula.

Question 23

What is the empirical formula?

Answer 23

It gives the simplest whole-number ratio of the atoms present in a compound.

Question 24

What is the molecular formula?

Answer 24

It shows the actual number of each type of atom present in a molecule or formula unit.

Question 25

How do you work out the empirical formula?

Answer 25

• Write down the mass of the element.
• Divide the mass by the mr.
• Divide that figure by the smallest figures found from those in the mr calculation.
• Find out the ratio.

Question 26

How do you work out the molecular formula from the empirical formula?

Answer 26

Find the Mr of the compound and x all numbers by the same figure appropriately.
Divide the molar mass of the compound by the empirical formula molar mass.