Question

What is the mass of 35.0 mL of hydrogen gas at STP?

Answer 1

Hint: To calculate the mass of any gas at STP you have to use the molar volume of the gas at STP. Further, you can use the molar volume to convert the gas in litres to moles. Now the moles of the gas can be converted to find the molar mass of the specific gas.

Complete answer:
To begin with, the molar volume of a gas is the volume occupied by the 1 mole of that respective gas under standard pressure and temperature conditions. The standard conditions usually are the temperature of 273 K and pressure of 1 atm. The common example of the molar volume of any gas at STP is 22.414 L/mol.
The given volume of Hydrogen gas is 35ml. This has to be converted to a standard system in litres. To convert it into litres it has to be divided by 1000.
$Volume\text{ }of\text{ }{{H}_{2}}\text{ in litres = }\dfrac{35}{1000}\text{ = 0}\text{.0350 L}$
Now, we will convert the volume of ${{H}_{2}}$in litres into moles. To convert the litres into moles divide the volume ${{H}_{2}}$ by the molar volume of gas.
$Moles\text{ of }{{\text{H}}_{2}}\text{ = }\dfrac{0.0350}{22.414}\text{ = 0}\text{.0015615 moles }$
Finally, to get the mass of the ${{H}_{2}}$ number of moles of ${{H}_{2}}$ gas in 0.0350 L has to be multiplied with the molar mass of ${{H}_{2}}$.
The molar mass of ${{H}_{2}}$= 2.016 gm.
$Mass\text{ of }{{H}_{2}}\text{ gas = 0}\text{.0015615 }\times \text{ 2}\text{.016 = 0}\text{.00315 gm}$
Final answer: The mass of 35.0 mL of hydrogen gas at STP = 0.00315 gm.

Note:
The molar volume of the gas is determined by using the ideal gas law equation that is \[PV=nRT\] . Using $\dfrac{V}{n}$ to find the molar volume of gas at any given temperature and pressure. Usually, the standard volume of gas at STP is used.