3.3

Calculations

11 flashcards to master Calculations

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Definition Flip

What units are used to measure concentration?

Answer Flip

Concentration can be measured in grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Mol/dm³ is also known as molarity (M).

Definition Flip

What is a mole and what is the Avogadro constant?

Answer Flip

A mole (mol) is the unit of amount of substance. One mole contains 6.02 x 10²³ particles (atoms, ions, molecules); this number is the Avogadro constant.

Key Concept Flip

What is the formula relating amount of substance (mol), mass (g), and molar mass (g/mol)?

Answer Flip

Amount of substance (mol) = Mass (g) / Molar mass (g/mol). This formula can be rearranged to solve for mass or molar mass.

Key Concept Flip

How do you calculate the number of particles in a given amount of substance?

Answer Flip

Number of particles = Amount of substance (mol) x Avogadro constant (6.02 x 10²³).

Example: 2 moles of carbon contain 2 x 6.02 x 10²³ carbon atoms.
Definition Flip

What is the molar gas volume at room temperature and pressure (r.t.p.)?

Answer Flip

At room temperature and pressure (r.t.p.), the molar gas volume is taken as 24 dm³. This means one mole of any gas occupies 24 dm³ at r.t.p.

Key Concept Flip

How do you calculate the mass of reactant needed to react completely with a given mass of another reactant?

Answer Flip

1. Write the balanced equation. 2. Convert masses to moles. 3. Use the stoichiometry of the balanced equation to find the moles of the other reactant. 4. Convert moles back to mass.

Example: 2Mg + O₂ -> 2MgO. How much Mg to react with 32g O₂? 32g O₂ = 1 mole O₂. So we need 2 moles Mg, that is 2 * 24 = 48g.
Key Concept Flip

What is a limiting reactant and how do you identify it?

Answer Flip

The limiting reactant is the reactant that is completely used up in a reaction. To identify it, calculate the moles of each reactant and compare the mole ratio to the balanced equation. The reactant with the smallest mole ratio (compared to the balanced equation's requirement) is the limiting reactant.

Key Concept Flip

How do you calculate the concentration of a solution in g/dm³ from mol/dm³ and vice-versa?

Answer Flip

To convert from mol/dm³ to g/dm³, multiply the concentration in mol/dm³ by the molar mass of the solute. To convert from g/dm³ to mol/dm³, divide the concentration in g/dm³ by the molar mass of the solute.

Key Concept Flip

Describe the steps in calculating the empirical formula from percentage composition.

Answer Flip

1. Assume 100g sample (percentages become masses in grams). 2. Convert grams to moles by dividing by the atomic mass. 3. Divide each mole value by the smallest mole value obtained. 4. If necessary, multiply to get whole numbers. These whole numbers are the subscripts in the empirical formula.

Example: Compound contains 40% C, 6.7% H, 53.3% O. Empirical formula = CH₂O.
Key Concept Flip

How do you calculate percentage yield?

Answer Flip

Percentage yield = (Actual yield / Theoretical yield) x 100%. The actual yield is the amount of product obtained experimentally. The theoretical yield is the amount of product calculated from the balanced equation assuming complete reaction.

Key Concept Flip

How do you calculate percentage purity?

Answer Flip

Percentage purity = (Mass of pure substance / Mass of impure substance) x 100%. This is used to determine the percentage of the desired compound in an impure sample.

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3.2 Equations 4.1 Electrolysis

Key Questions: Calculations

What units are used to measure concentration?

Concentration can be measured in grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Mol/dm³ is also known as molarity (M).

What is a mole and what is the Avogadro constant?

A mole (mol) is the unit of amount of substance. One mole contains 6.02 x 10²³ particles (atoms, ions, molecules); this number is the Avogadro constant.

What is the molar gas volume at room temperature and pressure (r.t.p.)?

At room temperature and pressure (r.t.p.), the molar gas volume is taken as 24 dm³. This means one mole of any gas occupies 24 dm³ at r.t.p.

About Calculations (3.3)

These 11 flashcards cover everything you need to know about Calculations for your Cambridge IGCSE Chemistry (0620) exam. Each card is designed based on the official syllabus requirements.

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