Question

Using the atomic weights of selected elements given below, find the formula weights for the following compounds:  

Atomic weights of selected elements

Ca = 40 g moL^-1, C = 12 g moL^-1, O = 16 g moL^-1, H = 1 g moL^-1, P = 31 g moL^-1, K = 39 g moL^-1                   Si = 28 g moL^-1, Al = 27 g moL^-1, S = 32 g moL^-1, N = 14 g moL^-1

Example: (NH₄)₂SO₄

Element	no. of moles	Atomic wt. (g moL-1)	no. moles x Atomic wt.
S	1	32	32
O	4	16	64
N	2	14	28
H	8	1	8
Formula wt.= Σ(no. moles x Atomic wt.) =			132

Compound	Formula weight (g moL-1)
(1pt) CaCO3
(1pt) Ca(OH)2
(1pt) CaO
(1pt) P2O5
(1pt) K2O
(1pt) SiO2 (quartz)
(2pt) KAl3Si3O10(OH)2 (muscovite)
(1pt) CO2

Answer 1

Solution :-

Formula weight is the sum of the atomic weights of the elemetns in the formula

Lets calculate the formula weight of the each compound using the given formula weight

1). CaCO3 = (Ca*1)+(C*1)+(O*3)

            = (40 *1) +(12*1)+(16*3)

        = 100 g/mol

2) Ca(OH)2 = (Ca*1)+(O*2)+(H*2)

                     =(40*1)+(16*2)+(1*2)

                    = 74 g /mol

3) CaO = (Ca*1)+(O*1)

             =(40*1)+(16*1)

              = 56 g /mol

4) P2O5 = (P*2)+(O*5)

               = (31*2) +(16*5)

               = 142 g/mol

5)K2O = (K*2)+(O*1)

            = (39*2)+(16*1)

            = 94 g/mol

6) SiO2 = (Si*1)+(O*2)

              = (28*1)+(16*2)

             = 60 g /mol

7) KAl₃Si₃O₁₀(OH)₂ = (K*1)+(Al*3)+(Si*3)+(O*12)+(H*2)

                                 = (39*1)+(27*3)+(28*3)+(16*12)+(2*1)

                                = 398 g/mol

8) CO2 = (C*1)+(O*2)

              = (12*1)+(16*2)

             = 44 g/mol