1999 - JAMB Chemistry Past Questions and Answers - page 1

The balanced equation of the reaction is:

Pb(NO₃)_2(aq) + 2HCl_(aq) ---> PbCl_2(s) + 2HNO_3(aq)

From the balanced equation,

1 mole of Pb(NO₃)₂ gave 1 mole of PbCl₂

using, molar concentration = no of moles/volume in dm3

therefore, no of moles = molar concentration x volume in dm3

where, molar concentration of Pb(NO₃)₂ = 0.1M

volume of Pb(NO₃)₂ in dm3 = 0.2 dm3

therefore, no of moles of Pb(NO₃)₂ = 0.1M x 0.2 dm3 = 0.02 mol

but from the balanced equation of the reaction, 1mol of Pb(NO₃)₂ gave 1mol of PbCl₂,

therefore 0.02mol of Pb(NO₃)₂ will give 0.02mol of PbCl₂.

But using, no of moles = mass/molar mass,

therefore mass = no of moles x molar mass

where, no of moles of PbCl₂ = 0.02mol

molar mass of PbCl₂ = 207 + (2 * 35.5) = 278 g/mol

therefore, mass of PbCl₂ = 0.02mol * 278g/mol

= 5.56g

therefore, mass of PbCl₂ that precipitated out is 5.56g.

Working and Answer was provided by

56.00cm³ of a gas at S.T.P weighed 0.11g

Molar volume of the gas = 22.4dm3 = 22400cm3

molar mass = (22400 x 0.11)/56

molar mass = 2464/56 = 44

Vapour density = Molar mass/2 = 44/2 = 22

From P1/T1 = P2/T2 = 9/273 = 3/T2

T2 x 9 = 273 x 3

T2 = (273 x 3)/9

T2 = 91K

Percentage by mass of M = 20.22%

Percentage by mass of Cl = ?

100 - 20.22 = 79.78%

Mass given ration of M

M = 2022/27 = 0.75

Chlorine = 79.78/35.5 = 2.24

Divide by the smallest

M = 0.75/0.75 = 1

Cl = 2.24/0.75 = 3

∴ The formula = MCl₃