Q13: Which of the following best describes the relationship between pH and H+ ion concentration?

The dilution formula is $M_1V_1 = M_2V_2$.

$$M_1 = 2.0 \text{ mol/dm}^3$$ $$V_1 = 50 \text{ cm}^3$$ $$V_2 = 200 \text{ cm}^3$$

$$M_2 = \frac{M_1V_1}{V_2}$$ $$M_2 = \frac{(2.0 \text{ mol/dm}^3)(50 \text{ cm}^3)}{200 \text{ cm}^3}$$

$$M_2 = \frac{100 \text{ mol/dm}^3 \cdot \text{cm}^3}{200 \text{ cm}^3}$$ $$M_2 = 0.5 \text{ mol/dm}^3$$

The new concentration is B. 0.5 mol/dm³.

Q20:
The pH scale is a measure of the acidity or alkalinity of a solution.
The pH scale ranges from D. 1 to 14. (Note: While 0-14 is common, 1-14 is also considered a valid range in some contexts, especially for general chemistry questions where 0 is extremely acidic and 14 is extremely basic). Given the options, 1 to 14 is the most appropriate choice.

Q21:

The dilution formula is $M_1V_1 = M_2V_2$.

$$M_1 = 0.5 \text{ mol/dm}^3$$ $$M_2 = 0.1 \text{ mol/dm}^3$$ $$V_2 = 500 \text{ cm}^3$$

$$V_1 = \frac{M_2V_2}{M_1}$$ $$V_1 = \frac{(0.1 \text{ mol/dm}^3)(500 \text{ cm}^3)}{0.5 \text{ mol/dm}^3}$$

$$V_1 = \frac{50 \text{ mol/dm}^3 \cdot \text{cm}^3}{0.5 \text{ mol/dm}^3}$$ $$V_1 = 100 \text{ cm}^3$$

The original volume was B. 100 cm³.

Q22:
To distinguish between a weak acid and a strong acid, an indicator that shows a range of pH values is needed.
Litmus paper only indicates if a solution is acidic or basic, not the strength.
Phenolphthalein changes color in a basic range (pH 8.2-10), making it unsuitable for distinguishing between acids.
Universal indicator paper changes color over a wide pH range, allowing for a more precise estimation of pH and thus distinguishing between strong and weak acids.
The most suitable indicator is C. Universal indicator paper.

Q23:

$$Ca = 40$$ $$C = 12$$ $$O = 16$$

$$\text{Molar mass of CaCO}_3 = (1 \times \text{Ca}) + (1 \times \text{C}) + (3 \times \text{O})$$ $$\text{Molar mass of CaCO}_3 = (1 \times 40) + (1 \times 12) + (3 \times 16)$$ $$\text{Molar mass of CaCO}_3 = 40 + 12 + 48$$ $$\text{Molar mass of CaCO}_3 = 100 \text{ g/mol}$$

The molar mass of calcium carbonate is C. 100 g/mol.

Q24:
When a strong acid is diluted by adding water, the concentration of hydrogen ions ($H^+$) decreases.
A decrease in $H^+$ concentration means the solution becomes less acidic.
Since pH is inversely related to acidity (lower pH means higher acidity), a decrease in acidity will cause the pH to increase.
The pH will B. increase (become less acidic).