# What is the molecular mass of a compound if 560mL have a mass of 1.10g at STP?

##### 2 Answers

The molar mass (molecular mass in grams) is

#### Explanation:

Note: This is going to be a long answer.

**First determine the number of moles.**

**Use the ideal gas law.**

where

**Given/Known**

**Unknown**

moles, n

**Equation**

**Solution**

Rearrange the equation to isolate

**Determine the molecular (molar) mass.**

**Given/Known**

**Unknown**

Molar mass:

**Equation**

**Solution**

Rearrange the equation to isolate

#### Explanation:

The idea here is that you need to use the fact that at **STP conditions**, *one mole* of any ideal gas occupies exactly

This means that if you know the volume of the gas, you can determine how many moles the sample contains by using the known molar volume at STP

#560color(red)(cancel(color(black)("mL"))) * (1color(red)(cancel(color(black)("L"))))/(1000color(red)(cancel(color(black)("mL")))) * "1 mole"/(22.7color(red)(cancel(color(black)("L")))) = "0.02467 moles"#

The molar mass of the compound, which tells you what the exact mass of *one mole* of the gas is, can be determined by using the mass of the sample.

#M_"M" = m/n#

#M_"M" = "1.10 g"/"0.02467 moles" = "44.59 g/mol"#

Rounded to two sig figs, the nswer will be

#M_"M" = color(green)("45 g/mol")#