5.2 - Quantitative chemistry

What is relative atomic mass (Ar)?

The average mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom.

Why can an element's Ar be a decimal?

It is an average that takes account of the relative abundance of its isotopes.

What is relative formula mass (Mr)?

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

How do you calculate Mr?

Add the Ar values of every atom shown in the formula.

What is the Mr of H₂O?

18.

What is the Mr of CO₂?

44.

What does conservation of mass mean?

No atoms are created or destroyed during a chemical reaction.

Why must chemical equations be balanced?

The same number of each type of atom must be present before and after the reaction.

What happens to total mass in a closed system during a reaction?

It stays the same.

Why can mass appear to decrease in an open reaction?

A gas may escape into the surroundings.

Why can mass appear to increase in an open reaction?

A gas from the surroundings may react with the substance.

What is a mole?

A unit used to measure the amount of a substance.

How many particles are in one mole?

6.02 × 10²³ particles.

What is Avogadro's constant?

6.02 × 10²³ particles per mole.

What is the equation for moles?

Moles = mass ÷ Mr.

How do you calculate mass from moles?

Mass = moles × Mr.

How do you calculate Mr from mass and moles?

Mr = mass ÷ moles.

What unit is mass usually measured in for mole calculations?

Grams (g).

What does 1 mole of a substance contain?

6.02 × 10²³ particles of that substance.

What does the balanced equation tell you about reacting substances?

The ratio in which their particles or moles react.

In 2H₂ + O₂ → 2H₂O, what is the mole ratio H₂:O₂?

2:1.

In 2H₂ + O₂ → 2H₂O, what is the mole ratio O₂:H₂O?

1:2.

Why are mole ratios useful?

They allow you to calculate the amount of one substance needed or produced from another.

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 that can form.

What happens to a reactant that is not limiting?

Some of it remains after the reaction.

How can you identify the limiting reactant?

Calculate how much product each reactant could produce; the one producing the least is limiting.

What is concentration?

The amount of solute dissolved in a given volume of solution.

What is the equation for concentration in g/dm³?

Concentration = mass ÷ volume.

What is the equation for mass?

Mass = concentration × volume.

What is the equation for volume?

Volume = mass ÷ concentration.

What unit is concentration commonly measured in?

g/dm³.

What unit is volume usually measured in for this equation?

dm³.

How do you convert cm³ to dm³?

Divide by 1000.

How do you convert dm³ to cm³?

Multiply by 1000.

What is percentage yield?

The actual amount of product obtained compared with the maximum theoretical amount.

What is the percentage yield equation?

Percentage yield = actual yield ÷ theoretical yield × 100.

Why is percentage yield usually less than 100%?

Some reactants may not react, products may be lost or side reactions may occur.

What would a 100% yield mean?

The actual yield equals the theoretical yield.

Why is a high percentage yield desirable?

Less reactant is wasted and the process is more efficient.

What is atom economy?

A measure of how much of the reactants become the desired product.

What is the atom economy equation?

Atom economy = Mr of desired product ÷ total Mr of reactants × 100.

Why is high atom economy better?

Less waste is produced.

What is the difference between atom economy and percentage yield?

Atom economy considers the balanced equation; percentage yield considers how much product is actually obtained.

Can a reaction have high atom economy but low percentage yield?

Yes.

Why might a reaction have low atom economy?

It produces unwanted products as well as the desired product.

What volume does one mole of any gas occupy at room temperature and pressure (RTP)?

24 dm³.

What is the gas volume equation at RTP?

Volume = moles × 24 dm³.

How do you calculate moles of a gas at RTP?

Moles = volume ÷ 24.

How many cm³ are in 1 dm³?

1000 cm³.

What volume does 1 mole of gas occupy at RTP in cm³?

24,000 cm³.

How do you calculate the mass of a product from a reactant?

Convert reactant mass to moles → use the mole ratio → convert product moles to mass.

How do you calculate the volume of a gas produced?

Convert the reactant to moles → use the mole ratio → multiply gas moles by 24 dm³.

How do you calculate concentration from a titration?

Use the known concentration and mole ratio to calculate moles of the unknown, then divide by its volume.

Why must volumes be converted into dm³ for concentration calculations?

The concentration equation uses dm³.

Why can percentage yield never normally exceed 100%?

You cannot obtain more product than the theoretical maximum; a value above 100% usually indicates an error or impurities.

Why might the actual yield be lower than the theoretical yield?

Reactants may not completely react, products may be lost during separation, or side reactions may occur.

Why is atom economy important in industry?

It reduces waste, lowers raw-material costs and makes processes more sustainable.

Why is percentage yield important in industry?

A higher yield means more useful product is obtained from the same reactants.

What is the key difference between yield and atom economy?

Yield measures actual product obtained; atom economy measures the proportion of reactants theoretically becoming the desired product.