Question

1. the molecular formula of sulphuric acid is H2SO4: (a) calculate the molecular mass of sulphuric acid. (b) how many moles are there in 9.8g of sulphuric acid ? (c) how many "gram-atoms" of each element are there in 9.8g of sulphuric acid ? (atomic masses: H = 1, S = 32, O = 16) 2. Find the number of molecules present in a drop of chloroform (CHCl3) weighing 0.0239g. (C = 12, Cl = 35.5, H = 1)

Answer 1

1) a) Molecular mass of sulphuric acid (H₂SO₄) = 2$\times$1 + 32 + 16$\times$4 = 98 u

b) Given mass of H₂SO₄ = 9.8 g
Number of moles (n) of H₂SO₄ = $\frac{\text{Given mass}}{\text{Molar mass}}$
n = $\frac{9.8}{98}$ = 0.1 moles

c) 98 g of H₂SO₄ contains = 32 g of sulphur
9.8 g of H₂SO₄ contains = $\frac{32}{98}\times 9.8$ = 3.1 gram sulphur atoms
98 g of H₂SO₄ contains = 2 g of hydrogen atoms
9.8 g of H₂SO₄ contains = $\frac{2}{98}\times 9.8$ = 0.2 gram hydrogen atoms
98 g of H₂SO₄ contains = 64 g of oxygen atoms
9.8 g of H₂SO₄ contains = $\frac{64}{98}\times 9.8$ = 6.39 gram oxygen atoms

2) Molar mass of chloroform (CHCl₃) = 12 + 1 + 3$\times$35.5 = 119.5 g mol^-1
119.5 g of CHCl₃ contains = 6.022$\times$10²³ molecules
0.0239 g of CHCl₃ = $\frac{6.022\times {10}^{23}}{119.5}\times 0.0239$ = 0.0012 molecules