Question

20g of NaCl is dissolved in water tp form a 450 ml solution.find its molefraction and molarity?

Answer 1

`chi_"NaCl"=2.67xx10^-3`

Explanation:

`chi_"NaCl"="Moles of salt"/"Moles of salt AND moles of water"`

`=((20*g)/(58.44*g*mol^-1))/(((20*g)/(58.44*g*mol^-1))+((450*g)/(18.01*g*mol^-1)))=2.67xx10^-3`

And for a two component solution....`chi_"water"=1-2.67xx10^-3`

`-=0.9973`.

For `"molarity"`, we assume that we use `450*mL` of water to prepare the solution, i.e. a `450*g` mass.

`"Molarity"="Moles of solute"/"Volume of solution"`

`=((20*g)/(58.44*g*mol^-1))/(0.450*L)=0.761*mol*L^-1`...here, the concentration is quoted with units as is appropriate for a molar concentration, whereas `chi` is DIMENSIONLESS.

Again, we assume that the volume of the solution is `0.450*L`.

Why are these operations an example of chemical change?