How to Find the Percent Composition by Mass for H2O (Water)

To find the percent composition (by mass) for each element in H2O we need to determine the molar mass for each element and for the entire H2O compound.

We’ll use the following formula for percent composition by mass:

Percent Composition by Mass = (Molar Mass of Element x Number of that Element)/ Molar Mass of Compound

In this video I’m using a Periodic Table that rounds to two decimal places. You may be asked to use a Periodic Table with more decimal places, however the process to find the percent composition by mass is the same.

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Note that if you add up the percentages for all of the elements it should be close to 100%. Often it will not be exact to the rounding of the atomic mass for elements.

Note: H2O is also known as Water.

[kfpriego]

Thank you! This helped me answer our homework!

[HayDayEveryday]

Actually in MCAT a question came find the mass ratio of H and O in water according to law of definite proportion..

[saadkhan]

really thank you help me in my last minute of exam

[acceniza]

Hello Dr. B, I know that you can't see this because its been 2 years past since you post this video but I'm still hoping that you can see this. I just want to ask, O = 16.00/18.02x100 = 88, 79023307 . that's the result when I solve it and also I used calculator to check if its correct, Can you explain for me why its not the same in the video that you posted. THANK YOU Dr. B - Your Student Ac

[alphaofficial]

But sir why can't I take molar mass of h atom only 1 if I take mm of h=1 then values for h=11.11% and O=88.88% is coming so am I wrong over here.pls correct I am confused😞😞😞

[HayDayEveryday]

And the answer was 11.11% and 88.89% ..correct! But they didn't ask for percentage composition they just said mass ratio

[IQYounus]

But can u pls tell so how to find the molar mass of a compund?

[rokychowdhury]

Thank you so much ❤
Bangladesh <2023