the natural abundance of two stable isotopes of hyrogen(hydrogen and deuterium) are 99.985% and 0.015%.?

Question

Assume the water exists as either H2O or D2O. Calculate the number of D2O molecules in exactly 400mL of water (Density=1.00g/mL)

Answer 1

Ok

percent means the number is 100 times larger than it really is.

of 400ml of water, 0.015% is of the deuterium isotope.
400ml * (0.00015) = .06 mL
Density is 1g/mL So 0.06mL*1g/mL is 0.06g D20

Water is H20, H is 1 amu, and oxygen is 16 amu. Deuterium is 2 amu ( one proton and one neutron 1+1=2).
So water has H+H+O, 1+1+16=18amu per mole
D20 is D +D +O, 2 +2+16 = 20amu per mole
Avogadro's number 6.02x10 +23 molecules per mole.

(0.06g D20) / (20 amu per mole D20) = moles D20= 0.003 Moles D2O.

Moles D20 * Avogadro's number molecules per mole = number of molecules D20
(6.02*10**23 Molecules per Mole) * ( 0.003 Moles) = molecules
=1806x10 +18 or 1.806x10 +21 Molecules