# chem percent composition

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• Max, I was having trouble with my homework last night on percent composition. Do you think you could help?﻿
• Sure Abby! Let's do a practice problem with Magnesium Chloride.
• Next we need to find the molar mass for each element, which is the mass on the periodic table.
• Alright, so the first thing we need to do is write out the correct formula it is MgCl2﻿
• Keep in mind since there are 2 of chlorine you will need to multiply the mass number by 2!﻿
• Mg:1*24.3g/mol Cl:2*35.5glmol﻿
• ﻿Mg molar mass: 24.3 g/mol Cl molar mass: 71g/mol﻿
• ﻿Now we do the part over whole right? So we add the 2 masses together to get 95.3 g/mol﻿?
• ﻿So we do 24.3/95.3 then you multiply that by 100 to get 25.5% which is Magnesium's percent composition﻿.
• A clue is that your percents at the end should add up close to 100﻿
• Which gives you 74.5% as the composition﻿.
• ﻿Abby exactly right! Then you would do the same thing for Chlorine.﻿ 71/95.3 *100.
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