Question

(a) How many moles of ammonium ions are in 0.795 g of ammonium carbonate?

(b) What is the mass, in grams, of 0.0507 mol of iron(III) phosphate?

(c) What is the mass, in grams, of 3.98*10^23 molecules of aspirin, C9H8O4?

(d) What is the molar mass of a particular compound if 0.090 mol weighs 6.38 g?

Answer 1

(a) Molar mass of ammonium carbonate,(NH₄)₂CO₃

                                 = 2[At.mass of N+(4xAt.mass of H)]+At.mass of C+(3xAt.mass of O)

                                 = 2[14+(4x1)]+12+(3x16)

                                 = 96 g/mol

Given mass of ammonium carbonate is 0.795 g

NUmber of moles , n = mass/molar mass

                              = 0.795g / 96 (g/mol)

                              = 828x10^-3 moles

1 mole of ammonium carbonate contains 2 moles of NH₄⁺ ions

828x10^-3 moles of ammonium carbonate contains 2x828x10^-3 moles of NH₄⁺ ions

We know that one mole contains 6.023x10²³ (avagadro number)ions

2x828x10^-3 moles of NH₄⁺ ion contains 2x828x10^-3 x6.023x10²³ = 9.97x10²¹ of NH₄⁺ ions

(b) Molar mass of iron(III) phosphate,FePO₄ = At.mass of Fe + At.mass of P + (4xAt.mass of O)

                                                                = 55.8 + 31 + (4x16)

                                                                = 150.8 g/mol

Number of moles , n = mass/molar mass

      Mass , m = number of moles x molar mass

                    = 0.0507 mol x 150.8 (g/mol)

                    = 7.645 g

(c) Molar mass of aspirin, C9H8O4 =(9xAt.mass of C)+(8xAt.mass of H) + (4xAt.mass of O)

                                                   = (9x12) + (8x1)+(4x16)

                                                   = 180 g/mol

We know that 1 mole of aspirin =180 g contains 6.023x10²³ molecules

6.023x10²³ molecules of aspirin weighs 180 g

3.98x10²³ molecules of aspirin weighs M g

M = (3.98x10²³ x 180) / (6.023x10²³ )

    = 118.9 g

(d) 0.090 mol of a compound weighs 6.38 g

      1.0 mol of the compound weighs M g

M = ( 1.0x6.38) / 0.090

    = 70.9 g

Therefore the molar mass of the compound is 70.9g/mol