Question

How many oxygen atoms are in 3.70 g of quartz?

Answer 1

We know that quartz is SiO₂(silica)

Molar mass of SiO₂ is = At.mass of Si + (2xAt.mass of O)

                                 = 28 + (2x16)

                                 = 60 g/mol

Given mass of SiO2 is , m = 3.70 g

So number of moles pf quartz , n = mass/molar mass

                                                = 3.70 / 60

                                                = 0.062 mol

1 mole of SiO₂ contains 2 moles of O

0.062 moles of SiO₂ contains 2x0.062 = 0.124 moles of O

Also we know that 1 mole of any compound contains 6.023x10²³ (avagadro number) of atoms.

0.124 moles of O contains 0.124x 6.023x10²³ = 7.468x10²² O atoms

2.00 g SiO2 ) * (1mol SiO2/60.1g)(2 mol O2/1 mol SiO2) ( 6.02*10^23 atoms / 1 mol o2) =4.01×10^22