calculate molar mass of water if it contains 50% heavy water{D₂O}?

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Solution

To solve we use a formula analogous to the formula relating to atomic mass of isotopes of elements to their relative abundance.
$M o l a r m a s s = m o l a r m a s s H_{2} O \times f r a c t i o n H_{2} O + m o l a r m a s s D_{2} O \times f r a c t i o n D_{2} O$
molar mass of H₂O = 18 g/mol fraction of H₂O in the sample = 50% = 0.5
molar mass of D₂O = 20 g/mol (contains one extra neutron than H for each of the two D atoms) fraction of D₂O = 0.5
Substituting the values,
molar mass of sample = 18 ^$\times$ 0.5 + 20 $\times$ 0.5 = 9 + 10 = 19 g/mol

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