# How to calculate percent composition by mass

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• ﻿Well sure. Finding percent composition can be very hard if you do not know what you are doing.
• Hey Mr. Smith can you explain how to calculate percent composition by mass.﻿
• First you just find what the molar mass of the compound you are measuring is. You find the molar mass of the substance by finding ﻿what elements are in the compound
• ﻿Once you know what elements are in the compound you must see how much of each element is in each singular molecule. Then you must find the atomic mass of each of those elements.
• From there you must multiply the atomic mass by how many atoms of the element there is in the molecule. Then you add up all of those products together.﻿ What have you have just found is the molar mass.
• ﻿Then you must go back to what element you are specifically measuring and find how much its atomic mass multiplied by the number of atoms is.  Then you simply divide this number by the molar mass and you have found you percent composition.
• What is the percent composition of oxygen in H2﻿O?   O=15.999 H=1.008  15.999+1.008+1.008=18.015 this is the molar mass 15.999/18.015=88.8 or 88.8%of the molar mass
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