Standard solutions of sodium hydroxide, NaOH, must be kept in airtight containers - VCE - SSCE Chemistry - Question 9 - 2016 - Paper 1

The reaction between carbonic acid (H₂CO₃) and sodium hydroxide (NaOH) leads to the formation of sodium carbonate (Na₂CO₃) and water. The balanced equation for this reaction is:

$$2 NaOH (aq) + H₂CO₃ (aq) \rightarrow Na₂CO₃ (aq) + 2 H₂O (l)$$

To find the moles of CO₂ that entered the container, we will use the ideal gas law equation:

$$PV = nRT$$

Where:

• P = 101.3 kPa (pressure of air)
• V = 9.90 L (volume of CO₂ entering)
• R = 8.31 L·kPa/(K·mol)
• T = 21.5 °C + 273 = 294.5 K (temperature in Kelvin)

First, we calculate the moles of air entering:

$$n_{air} = \frac{PV}{RT} = \frac{101.3 \text{ kPa} \times 9.90 \text{ L}}{8.31 \text{ L·kPa/(K·mol)} \times 294.5 \text{ K}} \approx 0.410 \text{ mol}$$

Next, we find the moles of CO₂ within that air volume knowing its concentration:

$$C_{CO₂} = 0.0400\% (v/v) = \frac{0.0400}{100} = 0.0004$$

Moles of CO₂ entering:

$$n_{CO₂} = C_{CO₂} \times n_{air} = 0.0004 \times 0.410 \approx 1.64 \times 10^{-4} \text{ mol}$$