chem percent composition

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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