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Formula mass and mole calculations

30 Mar Formula mass and mole calculations

Arguably the toughest chapter of Chemistry for many students around the world is the calculation of Mole. It is highly important for future scopes of chemistry and in the field of higher studies. But students often tend to avoid it without even trying to get along with it. This practice can prove to be dangerous in the long run and can be responsible for the downfall of grades of many students.

The topic that we are focusing on for today’s Chemistry tuition is the mole calculation and molar volume of gases. One thing that you should definitely memorize in the topic of Mole is the mole calculation. The equation shows how relative formula mass, number of moles and mass are related: number of moles = mass ÷ relative formula mass. This can be rearranged to find the mass if the number of moles and molar mass (its relative formula mass in grams) are known. It can also be rearranged to find the molar mass if the mass and number of moles are known. For example, calculate the mass of (a) 2 moles and (b) 0.25 moles of iron. (Relative atomic mass of Fe = 56).


a)Mass of 2 moles of iron
= number of moles x molar mass
= 2 x 56
= 112g

b) Mass of 0.25 mole of iron
= number of moles x molar mass
= 0.25 x 56
= 14g

One mole of any gas has a volume of 24 dm3 or 24,000 cm3 at rtp (room temperature and pressure). This volume is called the molar volume of a gas. This equation shows how the volume of gas in dm3 at rtp is related to the number of moles: volume of gas at rtp = number of moles × 24. For example, calculate the volume of carbon dioxide gas, CO2 , occupied 5 moles and (b) 0.5 moles of the gas occupied at STP.


a) Volume of CO2
= number of moles of CO2 x 22.4 L
= 5 x 22.4
= 112 L

b) Volume of CO2
= number of moles of CO2 x 22.4 L
= 0.5 x 22.4
11.2 L

The molar volume, symbol Vm, is the volume occupied by one mole of a substance (chemical element or chemical compound) at a given temperature and pressure. It is equal to the molar mass (M) divided by the mass density (p). It has the SI unit cubic metres per mole (m3 /mol), although it is more practical to use the units cubic decimetres per mole (dm3 /mol) for gases and cubic centimetres per mole (cm3 /mol) for liquids and solids.

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