Topic 1d - Calculations Involving Masses

Question 1

How do you calculate the relative formula mass (Mr)?

Answer 1

All the relative atomic masses (Ar) of the atoms in that compound added together

Question 2

What is the conservation of mass?

Answer 2

the total mass of reactants is equal to the total mass of the products

Question 3

What is the law of conservation of mass?

Answer 3

No atoms are lost or gained during a reaction, instead, the atoms rearrange into different combinations. This means that there are the same amount of each atom on both sides of the equation.

Question 4

What are the three possible results in a non-enclosed system (open system)?

Answer 4

• no change in mass because either no reaction occurs, or no gas is released or taken from the air
• the substances become ‘heavier’ because it takes something from the air
• the substances become ‘lighter’ because a gas is produced which escapes into the air

Question 5

What is an empirical formulae (EF)?

Answer 5

the simplest whole number ratio of atoms or ions of each element in a substance

Question 6

What is a molecular formulae (MF)?

Answer 6

represents the actual number of atoms of each element in one molecule of a substance

Question 7

How do you find the molecular formulae?

Answer 7

1) Find the Mr of the empirical formula using the Ar values of the elements that make up the compound.
2) Divide the Mr of the compound by the Mr of the empirical formula.
3) Multiply everything in the empirical formula by the result of step 2.

Question 8

What is a closed system?

Answer 8

Where nothing can get in or out

Question 9

What is an example of a closed system reaction?

Answer 9

The precipitation reaction:

Copper sulphate solution reacts with sodium hydroxide to form an insoluble copper hydroxide and soluble sodium sulfate.
As no reactants or products can escape, the scales would read the same throughout the experiment.

Question 10

What is the similarity between the products and reactants, in terms of the Mr?

Answer 10

The Mr of the reactants will be the same as the Mr of the products

Question 11

Why might the mass of something inside a container increase during a reaction?

Answer 11

• The particles in a gas move around and fill the space they’re in
• When the gas reacts to form part of the product, the particles become contained inside the reaction vessel — so the total mass of the stuff inside the reaction vessel increases.

Question 12

Why might the mass of something inside a container decrease during a reaction?

Answer 12

Before the reaction, all the reactants are contained in the reaction vessel. If the vessel isn’t enclosed, then the gas that’s produced can escape from the reaction vessel as it’s formed. It’s no longer contained in the reaction vessel, so you can’t account for its mass - the total mass of the stuff inside the reaction vessel decreases.

Question 13

How do you calculate the empirical formulae?

Answer 13

1) Use the periodic table to find the A, of all the elements in the compound.
2) Divide the mass of each element in the compound by its relative atomic mass (A). This is to find the relative amount of each element.
3) Write the relative amount of each element as a ratio.
4) Find the smallest whole number ratio between the elements in the compound. To do this, divide each number in the ratio by the
smallest number.
5) Use the smallest whole number ratio to write the empirical formula.

or

You can also find the empirical formula of a compound from the percentage of each element that it contains (its percentage composition). The method for doing this is the same as the one above, but you divide the percentage (rather than the mass) of each element by its A,.

Question 14

How do you do the magnesium experiment to find the empirical formula?

Answer 14

1) Get a crucible and heat it until it’s red hot.
2) Leave the crucible to cool, then weigh it, along with its lid.
3) Add some clean magnesium ribbon to the crucible. Reweigh the
crucible, lid and magnesium ribbon. The mass of magnesium you’re using is this reading minus the initial reading for the mass of the crucible and lid.
4) Heat the crucible containing the magnesium. Put the lid on the crucible so as to stop any bits of solid from escaping, but leave a small gap to allow oxygen to enter the crucible.
5) Keep heating the crucible strongly for around 10 minutes, or until all the magnesium ribbon has turned white.
6) Allow the crucible to cool and reweigh the crucible with the lid and its contents. The mass of magnesium oxide you have is this reading, minus the initial reading for the mass of the crucible and lid.

Question 15

What is concentration?

Answer 15

The amount of a substance (e.g., mass) in a certain volume of a solution

Question 16

How do you calculate the concentration in terms of mass?

Answer 16

concentration = mass of solute ÷ volume of solution

Question 17

What are the units for concentration?

Answer 17

g dm^-3

Question 18

What are the units for mass of solute?

Answer 18

g

Question 19

What are the units for the volume of solution?

Answer 19

dm^3

Question 20

How do you calculate the mass of a product?

Answer 20

1) Write out the balanced equation.
2) Work out the relative formula masses (M) of the reactants and products that you’re interested in.
3) For the substance that you know the mass of, work out the relative amount of it in the reaction. To do this, divide its mass by its relative formula mass.
4) Use the balanced equation to work out the relative amount of the other substance.
5) Then multiply the relative amount of the substance you don’t know the mass of by its relative formula mass. This gives the mass of that substance in the reaction.

Question 21

What is a limiting reactant?

Answer 21

The reactant that’s used up - it limits the amount of products formed

Question 22

What is Avogadro’s constant?

Answer 22

6.02 x 10^23

Question 23

What is a mole?

Answer 23

A standard scientific unit for measuring large quantities of very small entities, such as atoms, molecules or other particles

Question 24

What is the mass of one mole of atoms the same number of grams as?

Answer 24

Relative atomic mass or relative formula mass

Question 25

What is the mole and Avogadro’s constant formula?

Answer 25

amount in moles = number of particles ÷ Avogadro’s constant

Question 26

What is the similarity between the relative mass and moles?

Answer 26

relative mass = 1 mole = 6.02 x 10^23 particles

Question 27

What is the mass and mole equation?

Answer 27

amount in moles = mass (g) ÷ relative mass

Question 28

How can you find the number of moles in a balanced equation?

Answer 28

The ‘big’ number in front of an atom is the number of moles

Question 29

How do you balance equations using reacting masses?

Answer 29

1) Divide the mass of each substance by its relative formula mass to find the number of moles.
2) Divide the number of moles of each substance by the smallest number of moles in the reaction.
3) If any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers.
4) Write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas.