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%100\%.

So it is easy to find out the formula:

Mass Percentage =msolutemsolution×100%\text{Mass Percentage }=\frac{m_{solute}}{m_{solution}}\times100\%

A 200g200g beer contains 20g20g alcohol. So, the mass percentage of alcohol in the beer is:

20g200g×100%=10%\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/gg/g. However, try not to use that to prevent a confusion with Solubility.

Now, there is an example:

I want to dilute a 50g50g, 98%98\% H2SO4H_2SO_4 to a 20%20\% solution. How much water should I add in grams to do this?

Solution:

50g50g is the mass of solution.

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

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

Note that the mass of H2SO4H_2SO_4 does not change before and after the water added.

So, let the fixed mass of H2SO4H_2SO_4 be cc and the mass of water should add be xx

Before adding water,

cg50g×100%=98%\frac{c g}{50g}\times100\%=98\%

c=98100×50c=\frac{98}{100}\times50

After adding water,

cg(50+x)g×100%=20%\frac{c g}{(50+x)g}\times100\%=20\%

c=20100×(50+x)c=\frac{20}{100}\times(50+x)

Compare the two equations we have,

98100×50=20100×(50+x)\frac{98}{100}\times50=\frac{20}{100}\times(50+x)

x=195gx=195g

So, the amount of water we should add is 195g195g.

Note by Christopher Boo
5 years, 8 months ago

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1 vote

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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.

Christopher Boo - 5 years, 8 months ago

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Thanks for this note...Could you also start a series on Organic Chemistry, if possible. Looking forward to your note on mole concept.

Anish Puthuraya - 5 years, 8 months ago

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

Christopher Boo - 5 years, 8 months ago

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NICE POST!CAN YOU GIVE SOME NOTES ON EQUILIBRIUM.

OJAS JAIN - 5 years, 8 months ago

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

Christopher Boo - 5 years, 8 months ago

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What does 20%20\% solution mean?

Daniel Lim - 5 years, 8 months ago

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Sorry, it is 20%

Daniel Lim - 5 years, 8 months ago

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In this particular example, it means that the mass of H2SO4H_2SO_4 is 2020% of the mass of the solution.

Anish Puthuraya - 5 years, 8 months ago

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