Tekanan uap suatu zat adalah tekanan yang ditimbulkan oleh uap jenuh zat itu. Tekanan uap larutan merupakan hasil antara Tekanan uap pelarut murni (P°) dikali fraksi mol pelarut (Xpelarut).

Berikut adalah penentuan tekanan uap larutan NaCl:

Kadar NaCl 10 % artinya 10 g NaCl dan 90 g air.

table attributes columnalign right center left columnspacing 0px end attributes row nt equals cell fraction numerator massa space Na Cl over denominator Mr space Na Cl end fraction end cell row blank equals cell fraction numerator 10 space g over denominator 58 comma 5 space g forward slash mol end fraction end cell row blank equals cell 0 comma 17 space mol end cell row np equals cell fraction numerator massa space H subscript 2 O over denominator Mr space H subscript 2 O end fraction end cell row blank equals cell fraction numerator 90 space g over denominator 18 space g forward slash mol end fraction end cell row blank equals cell 5 space mol end cell row blank blank blank row cell P subscript larutan end cell equals cell space Xp space x space P degree end cell row blank equals cell fraction numerator np over denominator np and nt space end fraction space x space P degree end cell row blank equals cell fraction numerator 5 space mol over denominator 5 space mol plus 0 comma 17 space mol end fraction x space 24 space mmHg end cell row blank equals cell 23 comma 21 space mmHg end cell end table

Jadi, tekanan uap larutan NaCl adalah 23,21 mmHg.