The mole fraction of $NaCl$, in a solution containing 1 mole of $NaCl$ in 1000g of water:
A. 0.0177
B. 1.001
C. 0.5
D. 0.244

Hint: For solving this question, first we have to find the ratio of (No. of moles of $NaCl$) to (No of moles of whole solution). And then we have to use the formula for calculating mole fraction i.e;
Mole fraction=$\dfrac{{no.\,of\,moles\,of\,NaCl}}{{no.\,of\,moles\,of\,Solution}}$

Complete step-by-step answer:
No. of moles of ${H_2}O$ = \[\dfrac{{Weight}}{{Molecular.Mass}}\] =$\dfrac{{1000}}{{18}}$ ​ =55.55
 No. of moles of NaCl= 1

Mole fraction of NaCl=$\dfrac{{no.\,of\,moles\,of\,NaCl}}{{no.\,of\,moles\,of\,NaCl + no.\,of\,moles\,of\,{H_2}O}}$
                                                      =$\dfrac{1}{{55.55 + 1}}$
                                                      =0.01768$ \approx $ 0.0177
Thus option A “0.0177” is the correct answer to the problem.

Note: For solving this question, one should know the molecular mass of an element and should also know how to calculate the no. of moles.