Question

A flask weighs 50.26g when completely empty and weighs 50.94g when full of air at 35°c and 80cmHg. What is the density of air at STP if the capacity of the flask is 500cmcube

Answer 1

Answer on Question #73755, Physics / Molecular Physics | Thermodynamics

A flask weighs 50.26g when completely empty and weighs 50.94g when full of air at $35{}^{\circ}\mathrm{C}$ and 80cm Hg. What is the density of air at STP if the capacity of the flask is $500\mathrm{cm}^3$.

Solution

$$\rho = \frac {m}{V};$$

1) Find the mass of the air:

$$m = 50.94 - 50.26 = 0,68 \, (\mathrm{g}) = 68 \times 10^{-5} \, (\mathrm{kg})$$

2) Find the volume at STP

$$\frac {P _ {1} V _ {1}}{T _ {1}} = \frac {P _ {S T P} V _ {S T P}}{T _ {S T P}};$$

$P_{1}, V_{1}, T_{1}$ – parameters given in the task,

$$500 \, \mathrm{cm}^3 = 0.0005 \, \mathrm{m}^3$$ $$35{}^{\circ} \mathrm{C} = 308 \, \mathrm{K}$$ $$80 \, \mathrm{cm} \, \mathrm{Hg} = 800 \, \mathrm{mm} \, \mathrm{Hg}$$ $$V _ {S T P} = \frac {P _ {1} V _ {1} T _ {S T P}}{T _ {1} P _ {S T P}} = \frac {800 \times 0.0005 \times 273}{308 \times 760} = 4,67 \times 10^{-4} \, (\mathrm{m}^3);$$ $$\rho = \frac {68 \times 10^{-5}}{4.67 \times 10^{-4}} = 1.46 \, (\mathrm{kg/m^3})$$

Answer

The density of air at STP is $1.46 \, \mathrm{kg/m^3}$.

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