grade 12 University level Chemistry

Question

When a sample of a compound, XZO_3 (s), is strongly heated, 3.469 g of oxygen gas and 4.225 g of XZ_(s) are formed. When all of the XZ_(s) was mixed with AgNO₃ solution in a second reaction, all of the XZ_(s) reacted to form 10.36 g of AgZ_(s) and some XNO₃. Calculate the atomic masses of X and Z and identify X and Z.

Answer 1

Hi Allan,

                   heat

2XZO₃(s)    ------------> 2XZ(s) + 3O₂     (Balanced equation)

XZ(s) + AgNO₃ --------> AgZ(s) + XNO₃  (Balanced equation)

 

mass of Oxygen = 3.469 g, molecular mass of O₂ = 2*16 =32.00 g/mole

mole of Oxygen produced = 3.469g/32.00g/mole = 0.1084 mole

From the first reaction, 

2 moles of XZO₃ produces 2 moles of XZ and 3 moles of oxygen gas or 

Ratio of moles of XZ: O = 2:3

moles of XZ = (2/3)*0.1084 moles = 0.072267 moles

molecular mass of XZ = (Mx + Mz) g/mole, mass of XZ = 4.225 g

moles of XZ = mass of XZ/ molecular mass of XZ

0.072267 = 4.225/(Mx + Mz)

(Mx + Mz) = 4.225/0.072267 = 58.464

From the second reaction

1 mole of XZ reacted with 1 mole of AgNO₃ which produced 1 mole of AgZ and 1 mole of XNO₃

number of moles of AgNO₃ = number of moles of XNO₃ = number of moles of AgZ = 0.072267

Molecular mass of AgZ = (107.8682 +Mz) g/mole

mass of AgZ = 10.36 g

10.36/(107.8682 +Z) = 0.072267

10.36/0.072267 = 107.8682+Mz

143.357 = 107.8682 + Mz

143.357 - 107.8682 = Mz

35.489 = Mz, From above, Mx + Mz =58.464 ,

Mx = 58.464 - Mz

     = 58.464 - 35.489

     = 22.975

The atomic mass of X is 22.975 and the atomic mass of Z is 35.489. From the periodic table

X is sodium (Na) and Z is Chlorine (Cl)