3: Quantitative chemistry

What is the law of conservation of mass?

No atoms are lost or made in a chemical reaction so mass of products equals mass of reactants.

Why must chemical equations be balanced?

Because the number of atoms of each element must be the same on both sides.

What do coefficients (multipliers) in equations show?

They show the number of molecules or moles of each substance.

What do subscripts in chemical formulas show?

The number of atoms of each element in a compound.

What is relative formula mass (Mr)?

The sum of the relative atomic masses of all atoms in a compound.

What happens to total Mr in a balanced equation?

Total Mr of reactants equals total Mr of products.

How do you calculate percentage by mass?

(Mass of element ÷ Mr of compound) × 100.

Why might mass appear to change in a reaction?

Because a gas may enter or leave the system.

Why does mass increase when metals react with oxygen?

Oxygen from the air is added to the metal.

Why does mass decrease in thermal decomposition?

A gas (e.g. CO₂) escapes into the air.

What is uncertainty in measurements?

The range of possible values within which the true value lies.

How can uncertainty be estimated?

Using the range of results around the mean.

What is a mole?

The amount of substance containing 6.02 × 10²³ particles.

What is the Avogadro constant?

6.02 × 10²³ particles per mole.

How is molar mass related to Mr?

Molar mass in grams equals Mr.

Formula to calculate moles from mass?

Moles = mass ÷ Mr.

Formula to calculate mass from moles?

Mass = moles × Mr.

What does a balanced equation show in terms of moles?

The ratio of moles of reactants and products.

How do you calculate masses from equations?

Use mole ratios and Mr values.

How do you balance equations using moles?

Convert masses to moles and simplify to whole number ratios.

What is a limiting reactant?

The reactant that is completely used up first.

Why is the limiting reactant important?

It determines the maximum amount of product formed.

What is concentration (g/dm³)?

Mass of solute per volume of solution.

Formula for concentration (g/dm³)?

Concentration = mass ÷ volume.

What is percentage yield?

The percentage of the maximum possible product actually obtained.

Formula for percentage yield?

(Actual yield ÷ theoretical yield) × 100.

Why is yield less than 100%?

Losses

What is atom economy?

A measure of how much of the reactants form useful products.

Formula for atom economy?

(Mr of desired product ÷ total Mr of reactants) × 100.

Why is high atom economy important?

It reduces waste and is more sustainable.

What is concentration in mol/dm³?

Moles of solute per volume of solution.

Formula for molar concentration?

Concentration = moles ÷ volume.

What is the volume of 1 mole of gas at RTP?

24 dm³.

What is RTP?

Room temperature (20°C) and pressure (1 atm).

What is true about equal moles of gases?

They occupy the same volume at the same temperature and pressure.

How do you calculate gas volume from moles?

Volume = moles × 24 dm³.

How do you calculate gas moles from volume?

Moles = volume ÷ 24 dm³.