Question

The Sulphur content of cystine is $26.7\%$. Given that cysteine contains two Sulphur atoms, what is the molecular weight of cystine?

Answer 1

Cystine has the molecular formula ${(SC{H_2}CH(N{H_2})C{O_2}H)_2}$. It is a white solid that is slightly soluble in water. It is common in many foods such as eggs, meat, dairy products and whole grains as well as skin, horns and hair.

Complete answer:
We know, the atomic mass of Sulphur is $32$.
We are given in the question that there are two atoms of Sulphur, the atomic mass of two Sulphur atoms is $2 \times 32 = 64g$.
The Sulphur content in cystine is $26.7\%$, so we can say that $26.7g$ of Sulphur is present in $100g$ of cystine.
And if $64g$ of Sulphur is present, than the molecular weight of cystine is:
$\dfrac{{100 \times 64}}{{26.7}} = 239.7gmo{l^{ - 1}}$

Note:
The molecular weight of cystine can also be calculated using the periodic table of the elements to find atomic weights, we find the atomic weight of every element in the given formula and to calculate the molecular weight, we add the contributions from each atom. But it can only be done if we know the chemical formula of cystine.