Calculate the number of moles and the mass of gas in 3.40 L of He at STP.
A)0.152 mol, 0.608 g
B)76.2 mol, 305 g
C)0.152 mol, 0.0379 g
D)76.2 mol, 19.0 g
E)1.00 mol, 4.00 g

Question

Quizplus · Accepted Answer

At STP, one mole of any gas occupies 22.4 L of volume. Therefore, the number of moles of He in 3.40 L can be calculated as: 
moles = volume (L) / 22.4 L/mol = 3.40 L / 22.4 L/mol = 0.152 moles
To calculate the mass of He, we need to use its molar mass which is 4.00 g/mol. Therefore: 
mass = moles x molar mass = 0.152 mol x 4.00 g/mol = 0.608 g. 
Thus, the correct answer is A.

Calculate the Number of Moles and the Mass of Gas