Question

Number of O atoms in 7.3 x10^-3 g of CaSO4 x 2H2O

Answer 1

The number of O atoms in 7.3 x10 ^-3 g of CaSO4 x 2H2O are

= 1.535 x10^20 atoms

calculation

step 1: calculate the moles of CaSO4 . 2H2O

• moles = mass/molar mass

• molar mass is of CaSO4.H2O=Mw of Ca + S +O +H2O
• = 40+32 +( 4x16) +2( (1x2)+16)= 172 g/mol

• moles is therefore= 7.3 x10^-3 g/172 g/mol= 4.244 x10^-5 moles

find the moles of O

• since there are 6 atoms of O the moles of O= 6 x ( 4.244 x10^-5)= 0.000255 moles

by use of Avogadro's law constant find the number of O atom

• that is 1 mole = 6.02 x10 ^23 atoms

0.000255 moles=? atoms

• by cross multiplication

= 0.000255 moles x(6.02 x10^23 atoms)/ 1 mole= 1.535 x10^20 atoms