C5.3. Quantitative Chemistry

What is the law of conservation of mass?

No atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.

Why must chemical equations be balanced?

The same number of atoms of each element must be present before and after a reaction.

What does a large number in front of a formula show?

The number of molecules or formula units involved in the reaction.

What does a small subscript number in a formula show?

The number of atoms of that element in one molecule or formula unit.

What is relative formula mass (Mr)?

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

How do you calculate Mr?

Add together the relative atomic masses of all the atoms shown in the formula.

What does a balanced equation show about relative formula masses?

The total Mr of the reactants in the quantities shown equals the total Mr of the products in the quantities shown.

What is percentage by mass?

The percentage of a compound's total mass that is due to a particular element.

How do you calculate percentage by mass?

Percentage by mass = (total relative atomic mass of the element in the formula ÷ Mr of the compound) × 100.

Why might a reaction in an open system appear to lose mass?

A gas produced by the reaction may escape into the surroundings.

Why might a reaction in an open system appear to gain mass?

A substance such as a gas from the surroundings may react with the chemicals.

What is uncertainty?

The range of values within which the true value is expected to lie.

How can the range be used as a measure of uncertainty?

The range of repeated measurements about the mean can be used to estimate the uncertainty.

What is a mole?

A unit used to measure the amount of a substance.

What is the symbol for mole?

mol.

What is Avogadro's constant?

6.02 × 10²³ per mole.

What does one mole of a substance contain?

6.02 × 10²³ of the stated particles, such as atoms, molecules or ions.

What is the mass of one mole of a substance?

Its relative formula mass in grams.

What is the equation for calculating moles?

Moles = mass ÷ Mr.

How do you calculate mass from moles?

Mass = moles × Mr.

What can a mole measure?

Atoms, molecules, ions, electrons, formulae and equations.

What does the coefficient in a balanced chemical equation represent in terms of moles?

The ratio of moles of each substance involved in the reaction.

How can you calculate the mass of a reactant or product using a balanced equation?

Use the equation to determine the mole ratio, calculate the moles of the known substance, use the ratio to find the moles of the required substance, then convert the moles into mass.

How can you balance an equation using masses?

Convert the given masses into moles, divide by the smallest number of moles to obtain a simple whole-number ratio, then use the ratio as the balancing numbers.

What is a limiting reactant?

The reactant that is completely used up first and therefore limits the amount of product that can form.

What is an excess reactant?

A reactant that is present in more than the amount needed, so some remains after the reaction.

How does a limiting reactant affect the amount of product formed?

The amount of product is limited by the amount of the limiting reactant available.

What is concentration?

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

What unit is used for mass concentration?

g/dm³.

What is the equation for mass concentration?

Concentration = mass of solute ÷ volume of solution.

How do you calculate the mass of solute?

Mass = concentration × volume.

How do you calculate the volume of solution?

Volume = mass ÷ concentration.

What must the volume be measured in when calculating concentration in g/dm³?

dm³.

How do you convert cm³ to dm³?

Divide by 1000.

How do you convert dm³ to cm³?

Multiply by 1000.

How is concentration related to the mass of solute?

For a fixed volume, increasing the mass of solute increases the concentration.

How is concentration related to the volume of solution?

For a fixed mass of solute, increasing the volume decreases the concentration.