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# Chemistry - Mass Percentage

Problem Set

How do we express the concentration of a solute in a solution? We use Mass Percentage.

Mass Percentage is one of the way of representing the concentration of a solute in a solution. Mass percentage is calculated as the mass of a solvent divided by the total mass of the solution, multiplied by $$100\%$$.

So it is easy to find out the formula:

$$\text{Mass Percentage }=\frac{m_{solute}}{m_{solution}}\times100\%$$

A $$200g$$ beer contains $$20g$$ alcohol. So, the mass percentage of alcohol in the beer is:

$$\frac{20g}{200g}\times100\%=10\%$$.

Some other notes:

1. The mass percentage does not change with the temperature and air pressure.

2. There are other ways to express the concentration of a solute in a solution such as volume percentage, molality and amount-of substance concentration.

3. Some reference books express mass percentage as $$g/g$$. However, try not to use that to prevent a confusion with Solubility.

Now, there is an example:

I want to dilute a $$50g$$, $$98\%$$ $$H_2SO_4$$ to a $$20\%$$ solution. How much water should I add in grams to do this?

Solution:

$$50g$$ is the mass of solution.

$$98\%$$ is the mass percentage before adding water.

$$20\%$$ is the mass percentage after adding water.

Note that the mass of $$H_2SO_4$$ does not change before and after the water added.

So, let the fixed mass of $$H_2SO_4$$ be $$c$$ and the mass of water should add be $$x$$

$$\frac{c g}{50g}\times100\%=98\%$$

$$c=\frac{98}{100}\times50$$

$$\frac{c g}{(50+x)g}\times100\%=20\%$$

$$c=\frac{20}{100}\times(50+x)$$

Compare the two equations we have,

$$\frac{98}{100}\times50=\frac{20}{100}\times(50+x)$$

$$x=195g$$

So, the amount of water we should add is $$195g$$.

Note by Christopher Boo
2 years, 12 months ago

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I can't find any good image for this topic. If you found one, feel free to give me the link! For your information, I will not continue any notes involve solution such as molality, ideal solution, Raoult's Law etc. because this topics need the concept of mol. So, I will continue to those section after the idea of mol is presented. However, I will continue my chemistry notes in gas. · 2 years, 12 months ago

Thanks for this note...Could you also start a series on Organic Chemistry, if possible. Looking forward to your note on mole concept. · 2 years, 12 months ago

Oops, sorry I can't start Organic Chemistry because I am weak in that part. · 2 years, 12 months ago

What does $$20%$$ solution mean? · 2 years, 12 months ago

Sorry, it is 20% · 2 years, 12 months ago

In this particular example, it means that the mass of $$H_2SO_4$$ is $$20$$% of the mass of the solution. · 2 years, 12 months ago

NICE POST!CAN YOU GIVE SOME NOTES ON EQUILIBRIUM. · 2 years, 12 months ago