In this post , we will be checking out what is weight percent or mass percent,what is Weight percent or mass percent formula, how to convert **weight percent to mole fraction**

**Weight percent or mass percent** is defined as the weight of the solute in 100 gm of the solution

$Weight \; percent= \frac {mass \; of \; the \; solute}{mass \; of \; the \; solution} \times 100$

So , if m is the mass of Solute and M is the mass of solvent, then Weight percent formula will be

$Weight \; percent= \frac {m}{m+M} \times 100$

It is generally given as 30% NaCL by weight solution or 50% HCL by weight solution

**Example on Weight percent**

1. A solution is prepared by adding 3 g of a substance A to 17 g of water. Calculate the mass per cent of the solute.

**Solution**

$Weight \; percent= \frac {mass \; of \; the \; solute}{mass \; of \; the \; solution} \times 100$

$=\frac {3}{3+17} \times 100$

=15%

2. How many grams of 5.0% by weight NaCL solutions are required to 6.4 g NaCL?

**Solution**

A 5% by weight NaCL solutions contains 5 g of NaCL in 100 g of solution

So 1 g of NaCL will be contained in = 100/5 =20 g of solution

Therefore 6.4 g of NaCL will be present in = 20 X 6.4 = 128 g of solution

Alternate Method

$Weight \; percent= \frac {mass \; of \; the \; solute}{mass \; of \; the \; solution} \times 100$

$5= \frac {6.4}{x} \times 100$

or x = 128 g

**How to convert Weight percent to mole fraction**

Now if weight percent of the solution is known, we can calculate the mole fraction of the solution or the solvent easily as explained with below example

**Example**

Calculate the mole fraction of HCl and H_{2}O in a solution of HCL acid in water, containing 20% HCl by weight.

**Solution**

The solution contains 20 grams of HCL acid and 80 grams of water. Also,

Molar mass of HCl is 36.5 grams/mole.

Molar mass of water is 18 grams/mole.

Moles of HCl = 20/36.5 = .547 moles of HCl

Moles of water = 80/18 = 4.44 moles of H2O.

$ Mole \; Fraction \; of \; solute = \frac {no \; of \; moles \; of \; solute}{no \; of \; moles \; of \; solute + no \; of \; moles \; of \; solvent}$

$Mole \; fraction \; of \; HCl = \frac {.547}{.547 + 4.44}= .109$

$Mole fraction of H2O = \frac {4.44}{.547 + 4.44} = .891$

So in short, following steps need to be executed

- Find the weight of Solute and solvent from weight percent or mass percent
- Find the molecules mass of the solute and solvent
- Calculate the moles of solute and solvent using weight and molecular mass
- Apply the mole fraction formula to obtain the mole fraction

**Practice Questions**

- Find the mass percent if 10 g of X is dissolved in 80 g of Y?
- Calculate the mole fraction of HCl and H
_{2}O in a solution of HCL acid in water, containing 10% HCl by weight. - A solution is prepared by adding 2 g of substance to 18 g of water. Calculate the mass percent of solute
- 20% (w/W) NaOH solution . Calculate the Mole fraction of NaOH and water

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