Mass Percent

Chemistry 11

Law of Definite Proportions The elements in a given chemical

compound are always present in the same proportions by mass

The mass of an element in a compound, expressed as a percent of the total mass of the compound, is the element’s mass percent

Magnesium Oxide We completed a lab involving the

combustion of magnesium to create magnesium oxide

From theory, we know that the formula for magnesium oxide should be MgO

One of the groups discovered that the mass percents were as follows: Mg – 58% O – 42%

Magnesium Oxide What should the mass percent be? Molar Masses

Magnesium – 24.31g/mol Oxygen – 16.00g/mol Magnesium oxide – 40.31g/mol

%7.39%

%100/31.40

/00.16%

%3.60%

%100/31.40

/31.24%

O

O

Mg

Mg

mass

xmolg

molgmass

mass

xmolg

molgmass

Different Compounds – Same Elements Determine the mass

percent of the following two hydrogen/oxygen compounds: Water Hydrogen peroxide

Determine the mass percent of the following two carbon/oxygen compounds: Carbon dioxide Carbon monoxide

Different Compounds – Same Elements Determine the mass

percent of the following two hydrogen/oxygen compounds: Water

H – 11.2% O – 88.8%

Hydrogen peroxide H – 5.94% O – 94.1%

Determine the mass percent of the following two carbon/oxygen compounds: Carbon dioxide

C – 27.3% O – 72.7%

Carbon monoxide C – 42.9% O – 57.1%

Percent Composition From Mass Data A sample of a

compound has a mass of 48.72g. The sample is found to contain 32.69g of zinc and 16.03g of sulphur. What is the percentage composition of thie compound?

%90.32%

%10072.48

03.16%

%10.67%

%10072.48

69.32%

S

S

Zn

Zn

mass

xg

gmass

mass

xg

gmass

Practice Problems

Page 82 Questions 1-4