Chemical Calculations

Question 1

What are the two sorts of relative mass?

Answer 1

Relative atomic mass (Ar) and relative molecular mass (Mr)

Question 2

What is relative atomic mass?

Answer 2

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

Question 3

What is relative molecular mass?

Answer 3

The average mass of one molecule of a substance, as compared with 1/12 the mass of a carbon-12 isotope. Mr is found by adding the Ar of the atoms in the compound together.

Question 4

What is the mole?

Answer 4

The mole is a quantifier measure of atoms.

Question 5

How many atoms are in a mole?

Answer 5

Avogadro’s constant: 6 × 10^23. There are 6 × 10^23 atoms in one mole.

Question 6

How can we find mass from moles?

Answer 6

Mass = number of moles × relative mass

No. of moles = mass ÷ relative mass

Question 7

What is mole ratio and how do we determine mole ratio?

Answer 7

Mole ratio is just the idea of “how much of A will react with how much of B”, and is determined using the numbers beside the reactants in a chemical eq.

Question 8

When you are given data in a question, what is the first step to do?

Answer 8

CALCULATE THE NUMBER OF MOLES, THEN MOLE RATIO.

Question 9

Cubic decimetre (dm3) is often used as the unit of volume in chemistry. What is 1 dm3 equal to in litres, m3, and cm3?

Answer 9

1 dm3 = 1 litre = 0.001 m3 = 1000 cm3

Question 10

What units are used in calculating mass and volume respectively?

Answer 10

Grams (g) for mass, cubic decimetres (dm3) for volume.

Question 11

What is aqueous concentration?

Answer 11

It refers to the amount of chemical per unit volume of water. It is only used for aqueous solutions.

Question 12

How do we find molar concentration and mass concentration respectively?

Answer 12

Molar concentration (M) – mol / dm3
Mass concentration – g / dm3

Molar concentration = Number of moles / Volume
Mass concentration = Mass (grams) / Volume

Question 13

What is the principle of the volume of gases?

Answer 13

One mole of any gas occupies a volume of 24 dm3 at a temperature of 25 °C and pressure of 1 atm.

Question 14

What is the relation between the ratio of the volume of gases to their mole ratio?

Answer 14

Ratio of the volume of gases = equivalent to its mole ratio. At room temperature and pressure, 1 mole of any gas would occupy 24 dm3.

Question 15

What are limiting and excess reagents?

Answer 15

In chemical reactions, there is often a reactant leftover (excess reactant) while another is completely used (limiting reactant).

Question 16

How do we write out the answer for limiting and excess reagents?

Answer 16

“[number] mol of [substance] would require [number 2] mol of [substance 2] for a complete reaction. However, there is insufficient [substance 2] / leftover [substance 2] after the reaction. Hence, [substance 1 or 2] is limiting.]”

Question 17

What is percentage mass? How is it calculated?

Answer 17

The the mass of a component of a substance in relation to its total mass.

Percentage mass = relative mass of component / relative mass of entire substance × 100%

Question 18

What is percentage yield? How is percentage yield calculated?

Answer 18

Percentage yield is the mass of a component of a substance in relation to its total mass.

Percentage yield = average yield of experiment / calculated yield × 100%

Question 19

Why do we calculate percentage yield?

Answer 19

In an experiment, the actual measurement you record will not be the same as the calculated, theoretical yield. This is because there would be impurities in the reagents.

Question 20

What is percentage purity? How is it calculated?

Answer 20

Percentage purity calculates the purity of an impure ore or mineral. It is the mass of an actual chemical in relation to the total mass.

Percentage purity = real mass of chemical / total mass, including impurities × 100%

Question 21

How might we obtain the real mass of a chemical?

Answer 21

The number of moles of a product can be traced back to the number of moles of the reactant.

Question 22

What is empirical formula?

Answer 22

Empirical formula reflects the correct ratio between the various elements in simplest form. For example, C6H12O6 can be expressed as CH2O in an empirical formula.

Question 23

How do we find empirical formula?

Answer 23

Step 1: State the mass ratio of elements.
Step 2: Divide by relative atomic mass to get the mole ratio of the constituent atoms.
Step 3: Divide the ratio by the smallest mole.
Step 4: The numbers in the ratio you get are used for your empirical formula.
Step 5: Multiply the ratio to get whole numbers if there are decimals.

Question 24

How do we find molecular formulae from empirical formulae?

Answer 24

Step 1: Calculate Mr of empirical formula.
Step 2: Divide given Mr by the Mr previously calculated.
Step 3: Multiply the empirical formula by your solution.