**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:

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

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

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 \(x\)

Before adding water,

\(\frac{c g}{50g}\times100\%=98\%\)

\(c=\frac{98}{100}\times50\)

After adding water,

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

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

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 \(195g\).

## Comments

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TopNewestI 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. – Christopher Boo · 3 years, 4 months ago

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OrganicChemistry, if possible. Looking forward to your note on mole concept. – Anish Puthuraya · 3 years, 4 months agoLog in to reply

– Christopher Boo · 3 years, 4 months ago

Oops, sorry I can't start Organic Chemistry because I am weak in that part.Log in to reply

What does \(20%\) solution mean? – Daniel Lim · 3 years, 4 months ago

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– Daniel Lim · 3 years, 4 months ago

Sorry, it is 20%Log in to reply

– Anish Puthuraya · 3 years, 4 months ago

In this particular example, it means that the mass of \(H_2SO_4\) is \(20\)% of the mass of the solution.Log in to reply

NICE POST!CAN YOU GIVE SOME NOTES ON EQUILIBRIUM. – Ojas Jain · 3 years, 4 months ago

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– Christopher Boo · 3 years, 4 months ago

Ok but that may take some time, hope you can wait because now I am focusing on gases. :)Log in to reply