In this experiment, you will determine the density of air. You uses a rigid flask fitted with a rubber bung and a valve. You connects the flask to a vacuum ...

(a) Recording measurements [1]

All three measurements from Table 11.1: mass of evacuated flask = 253.72 g, mass of flask + air = 254.34 g, volume of water to fill flask = 500 cm³.

(b) Mass of air [1]

\( m = 254.34 - 253.72 = 0.62 \) g

This small difference (less than 1 g) shows why a sensitive balance is essential for this experiment.

(c) Density of air [2]

\( \rho = \frac{m}{V} = \frac{0.62}{500} \) [1]

\( \rho = 0.00124 \) g/cm³ [1]

(d) Conversion to kg/m³ [1]

Since 1 g/cm³ = 1000 kg/m³:

\( 0.00124 \text{ g/cm}^3 \times 1000 = 1.24 \text{ kg/m}^3 \)

Alternatively: multiply by \( \frac{1 \text{ kg}}{1000 \text{ g}} \times \frac{(100 \text{ cm})^3}{(1 \text{ m})^3} = \frac{10^6}{10^3} = 1000 \).

(e) Percentage difference [1]

\( \% \text{ difference} = \frac{|1.24 - 1.2|}{1.2} \times 100 = \frac{0.04}{1.2} \times 100 = 3.3\% \)

Accept 2 to 5 % depending on rounding. A small percentage difference indicates the method is reasonably accurate.

(f) Why the flask must be rigid [1]

When the flask is evacuated, the external atmospheric pressure pushes inward. A flexible container would collapse, reducing its internal volume. When air is re-admitted, the volume of air entering would not equal the original internal volume, so the density calculation \( \rho = m/V \) would use the wrong value of \( V \).

(g) Why the pump may not remove all air [1]

No mechanical pump can achieve a perfect vacuum. A small amount of residual air always remains trapped in the flask, in the valve, or in dead spaces in the tubing. This means the measured mass of air (difference between the two weighings) is slightly less than the true mass of air that fills the flask, giving a density that is slightly too low.

(h) Improvement [1]

Any valid suggestion: repeat the experiment several times and calculate the mean density to reduce the effect of random errors; use a larger flask so the mass of air is greater, reducing the percentage uncertainty in the mass measurement; use a more sensitive balance; or use a better vacuum pump to remove more air.

(i) Temperature reading and its significance [1]

Temperature = 22 °C. The density of air depends on temperature: warmer air expands and becomes less dense, while cooler air is denser. Recording the temperature allows the result to be compared with the accepted value at the same temperature, and explains any discrepancy if conditions differ from the standard reference temperature.