# A volume of 281 mL of hydrogen is collected over water; the water level in the collecting vessel...

## Question:

A volume of 281 mL of hydrogen is collected over water; the water level in the collecting vessel is the same as the outside level. Atmospheric pressure is 756.0 Torr and the temperature is 25°C. Calculate the molar mass of the metal.

## Molar Mass:

The molar mass of an element is the mass of one mole of that element. It can briefly defined as the mass of Avogadro number of atoms or molecules of a substance. The unit of molar mass is g/mol.

Let's look at the given data to help up find the molar mass of the metal:

{eq}V = 281\ mL = 0.281 L\\ P = 756.0 \ Torr = 0.995\ atm\\ T = 25^o C = 298K {/eq}

The value of the universal gas constant is:

{eq}R = 0.0821\ L \ atm/ mol\ K {/eq}

We can use the ideal gas equation to find the number of moles of a gas. Then we can get the molar mass of the metal if its mass is known.

So, the number of moles is;

{eq}PV = nRT\\ 0.995 \times 0.281 = n \times 0.0821 \times 298\\ n = 0.011428\ moles {/eq}

So, the molar mass of the metal is:

{eq}\text{Number of moles } = \dfrac{\text{Mass}}{\text{Molar mass}}\\ 0.011428 = \dfrac{m}{M}\\ M = \dfrac{m}{0.011428}\ g/mol {/eq}