Question

A balloon containing helium gas expands from 230 mL to 860 mL as more helium is added. What was the initial quantity of helium present if the expanded balloon contains 3.8 × 10-4 mol, assuming constant temperature and pressure? Which of the variables are known?

Answer 1

beginning volume and ending quantity of gas

Answer 2

Using the Ideal Gas Law:


`pV=nRT\iff n=(pV)/(RT)`

Calculating the ratio between the initial and the final numbers of moles:


`(n_1)/(n_2)=((p_1V_1)/(RT_1))/((p_2V_2)/(RT_2))`

The statement says that
`T_1=T_2,~p_1=p_2`. Hence:


`(n_1)/(n_2)=(V_1)/(V_2)\iff (n_1)/(3.8*10^(-4))=(230)/(860)\iff\\\\ n_1=(3.8*10^(-4)*230)/(860)\iff\boxed{n_1\approx1.0*10^(-4)~mol}`