Hello,

For the problems of hardness of water mill equivalence concept have to be used.

Moles of cacl2=Moles of CaCO3

Therefore moles of caco3=30mg/111=.27 mmoles

which is equal to 27.02mg of caco3

Hardness =( wgt of caco3/wgt of water)*10^6

wgt of water=100g=100000mg(it is given 100ml)

hence hardness=27.02/100000*10^6=270.2ppm

here 1ppm=1 part of caco3 in 10^6 parts of water.

Hope it helped.