Mass calculations

Question 1

Calculating Empirical Formula

Answer 1

Steps:
1. Find the mass of each element.
2. Divide by the relative atomic mass (Ar).
3. Divide all results by the smallest number.
4. Write the ratio as the empirical formula.

Question 1

Relative Formula Mass (Mr)

Answer 1

Relative Formula Mass (Mr): Sum of the relative atomic masses of all atoms in a compound.
Example:
MrofH2O=(2×1)+16=18

Question 2

Empirical vs Molecular Formula

Answer 2

Empirical Formula: Simplest whole-number ratio of atoms in a compound.
Molecular Formula: Actual number of atoms in one molecule of a compound.
Example:
CH2O
O (Empirical) vs C6H12O6
(Molecular)

Question 3

Deduce Empirical Formula from Molecular Formula

Answer 3

Example: Molecular Formula
C4H10 → Empirical Formula = C2H5

(Divide by 2).

Question 4

Law of Conservation of Mass (Closed System)

Answer 4

Definition: Mass is conserved in a reaction; mass of reactants = mass of products.

Question 4

Deduce Molecular Formula from Empirical Formula

Answer 4

Steps:
1. Find the Mr of the empirical formula.
2. Divide the relative molecular mass by the empirical formula mass.
3. Multiply the subscripts in the empirical formula by this number.

Question 5

Experiment to Determine Empirical Formula

Answer 5

Method:
1. Heat a metal (e.g., magnesium) in excess oxygen.
2. Measure the mass before and after.
3. Use mass data to calculate the empirical formula (mass of metal and oxygen).

Question 6

Law of Conservation of Mass (Non-Enclosed System)

Answer 6

Non-Enclosed System: Mass may appear to change due to gas release or absorption (e.g., burning wood, CO₂ released).

Question 6

Number of Particles from Moles

Answer 6

Numberofparticles=Moles×6.02×10^23

Question 7

Moles from Mass

Answer 7

Formula;
Moles = Mass(g)
—————–
Relative Formula Mass(Mr)

Question 7

Calculating Mass of Product from Reactant

Answer 7

Steps:
1. Write the balanced equation.
2. Find the moles of reactant.
3. Use mole ratio to find moles of product.
4. Convert moles of product to mass.

Question 8

The Mole

Answer 8

Definition:
A mole is: 6.02×10^23
particles of a substance (Avogadro’s number).
Molar Mass: Mass of one mole of a substance in grams = Mr (e.g., 1 mole of H₂O = 18g).

Question 8

Calculating Mass of Reactant from Product

Answer 8

Steps:
1. Write the balanced equation.
2. Find the moles of product.
3. Use mole ratio to find moles of reactant.
4. Convert moles of reactant to mass.

Question 9

Concentration of Solution (g/dm³)

Answer 9

Formula:
Concentration(g/dm^3)= mass of solute(g)
————————–
Volume of solution(dm^3)

Question 10

Mass from Moles

Answer 10

Mass(g)=Moles×Mr

Question 11

Moles from Number of Particles

Answer 11

6.02 x 10^23

Question 12

Mass of Product and Limiting Reactants

Answer 12

Limiting Reactant: The reactant that is used up first, controlling the amount of product.
Mass of Product: Depends on the amount of limiting reactant.

Question 12

Deducing Balanced Equation

Answer 12

Steps:
Find the moles of each substance.
Simplify the mole ratio.
Write the balanced equation based on this ratio.