O-LevelPhysicsThermal Physics5054/22 · Oct/Nov 2014 · Q36 Marks

Fig. 3.1 shows a helium cylinder being used at a funfair to inflate balloons that float in the air. [Figure 3.1] Initially, all the helium is stored in the cylinder at a pressure of 2.0 × 10⁷ Pa. When no more balloons can be filled, all the helium occupies a total volume of 1.8m³ and is at a pressure of 1.0 × 10⁵ Pa. The temperature of the helium has not changed.

(a)Calculate, stating the formula used, the initial volume of the helium in the cylinder.[3]
(b)(i)The balloon seller takes the cylinder back to the helium supplier and it is refilled. The mass of helium in the cylinder is 0.30 kg. Calculate the density of helium in the cylinder.[2]
(b)(ii)The mass of the empty cylinder is 16kg. Suggest why weighing the cylinder is not an accurate method of determining the amount of helium supplied.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] V₁ = P₂V₂/p₁ or p ∝ 1/V
[1] 1.0 × 10⁵ × (1.8/2.0) × 10⁷
[1] 9.0 x 10⁻³ m³ or 9000 cm³
[1] (p = ) m/ V or (0.30/9.0) × 10⁻³
[1] 33(.3333) kg/m³ or 0.033(3333)g/cm³
[1] helium mass/weight small (fraction of total/mass of air included) or this includes the weight of the cylinder

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