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## Problem 1.6 Calculate the number of atoms in each of the following: - 52 moles of Argon (Ar) - 52 u of Helium (He) - 52 g of Helium (He) ### Solution 1. 52 moles of Argon - 1 mole of Argon atoms = 6.022 x 1023 atoms of Ar - 52 moles of Ar = 52 moles * (6.022 x 1023 atoms/mol) = 313.144 x 1023 at...

## Problem 1.6 Calculate the number of atoms in each of the following: - 52 moles of Argon (Ar) - 52 u of Helium (He) - 52 g of Helium (He) ### Solution 1. 52 moles of Argon - 1 mole of Argon atoms = 6.022 x 1023 atoms of Ar - 52 moles of Ar = 52 moles * (6.022 x 1023 atoms/mol) = 313.144 x 1023 atoms of Argon 2. 52 u of Helium - Atomic mass of He = mass of 1 atom of He = 4.0 u - 4.0 u of He = 52 u? = 52 / 4.0 = 13 atoms of He 3. 52 g of He - Mass of 1 mole of He = 4.0 g - Number of moles of He = mass of He / mass of 1 mole of He = 52 g / 4.0 g/mol = 13 mol - Number of atoms of He = Number of moles * 6.022 x 1023 atoms/mol = 13 mol * 6.022 x 1023 atoms/mol = 78.286 x 1023 atoms of He ## Problem 1.7 Calculate the number of moles and molecules of ammonia (NH3) gas in a volume of 67.2 dm3 of it measured at STP. ### Solution: - Volume of NH3 at STP = 67.2 dm3 - Molar volume of a gas = 22.4 dm3/mol - Number of moles (n) = Volume of the gas at STP / Molar volume of gas = 67.2 dm3 / 22.4 dm3/mol = 3.0 mol - Number of molecules = Number of moles * 6.022 x 1023 molecules/mol = 3.0 mol * 6.022 x 1023 molecules/mol = 18.066 x 1023 molecules ## 1.9 Moles and Gases Many substances exist as gases. If we want to find the number of moles of gas, we can do this more conveniently by measuring the volume rather than mass of the gas. Chemists have deduced from Avogadro law that "One mole of any gas occupies a volume of 22.4 dm3 at standard temperature (0°C) and pressure (1 atm) (STP). The volume of 22.4 dm3 at STP is known as molar volume of a gas.