Percent Composition Review

What is percent composition?

• The percent by mass of an element in a compound.

• The percent by number of atoms of an element in a compound.

• The mole ratio of an element in a compound.

• The mole ratio between the salt and water in a hydrate.

What is the equation for calculating percent composition?

c

d

mass of element

moles of compound

moles of an element

mass of compound

number of atoms of element

number of atoms in compound

Tags

NGSS.HS-PS1-7

What is the percent composition of copper in copper(II) bromide?

• 28.45%

• 33.33%

• 50.00%

• 44.30%

What is the percent composition of nitrogen in ammonium sulfide?

• 41.10%

• 20.55%

• 27.95%

• 18.18%

A sample of magnesium sulfate heptahydrate was measured to have a mass of 15.30 g. After the sample was heated to remove the water from the compound, it had a mass of 7.47 g. What is the percent water in the compound?

• 48.8%

• 51.2%

• 95.4%

• 105%

Answer explanation

• The mass remaining after being heated represents the mass of the magnesium sulfate.

• The mass of the water was lost when heated.

• You need to subtract the total mass by the remaining mass to calculate the mass of the water.

What is the percent composition of sulfur in aluminum sulfide?

What is the percent composition of potassium in potassium permanganate?

A chemist determines that 1.26 g of iron reacts with 0.54 g of oxygen to form rust. The percent composition of each element in the product is: ​ (a)   % iron and ​ (b)   % oxygen.

• 70.0
• 30.
• 0.3
• 2.33
• 42.9
• 57.1
• 0.70

Tags

NGSS.HS-PS1-7

A student is given a 11.00 g sample of sodium carbonate decahydrate (Na₂CO₃·10H₂O). After heating, the remaining anhydrate weighs 3.80 g. Find the percent water.

In the lab, a chemist analyzed a sample of methanol and found that it was made of 6.2 g of carbon, 4.1 g of hydrogen, and 15.9 g of oxygen. What is the percent composition of carbon in methanol?

Answer explanation

The law of conservation of mass indicates that the total mass of the products will equal that of the total mass of the reactants. Therefore, to get the total mass of the compound, you will need to add up the masses for each of the elements that make up the compound.