Answers

2015-03-17T14:58:32+05:30
40u contains 1 mole Ca 
0.4u contains (0.4/40 moles) Ca = 1/100 moles
1 mole of Ca contains 6.02*10^23 atoms
so 0.01 mole of Ca contains 6.02*10^21 atoms

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2015-03-17T15:06:25+05:30
The mass of single ca is 40u
so mass of 1 mole of ca is or mass of  6.023 x 10∧23 atome = 40u
and for 0.4 gram ca = 0.4/ 40 x 6.022 x 10∧23
                             = 6.022 x 10∧21

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