Percent by Mass

Percent by Mass

​Law of Definite Proportions

​The Law of Definite Proportions states that a given chemical compound always contains the same elements in the exact same proportions by mass.

​For example, whether I have a drop of water or a cup of water, the ratio of hydrogen to oxygen will always be the same.

​Compounds vs. Mixtures

​Compounds

• proportion​ of elements present is always the same

​

​Mixtures

• ​proportion of substances may very

• ​ie. the photos of coffee show the difference when a higher or lower content of coffee is present in the water.

Multiple Choice

The proportion of elements in a compound is always the same.

• True

• False

Multiple Choice

The proportion of substances in a mixture is always the same.

• True

• False

Calculating percentage of a substance in a compound or mixture​

YAY! Math!​

Grab your calculator and a paper and pencil before you move on!

​Mass Percent

Mass percent is found by taking (mass of the part)/(mass of the whole) x 100.

% Substance = (mass of the part) x 100

​(mass of the whole)

Multiple Choice

Brass is comprised of zinc and copper.

What is the mass percent of 15.4 g of zinc in a 23.3-g sample of brass?

• 66.1%

• 38.7%

• 7.9%

• 1.51%

Multiple Choice

Brass is comprised of zinc and copper.

What is the mass percent of 3.5 g of zinc in a 23.3-g sample of brass?

• 26.8%

• 15.0%

• 6.66%

• 19.8%

Multiple Choice

You have calculated the percent of zinc in two different brass samples.  Based on your answers, what type of matter is brass?

• element

• compound

• mixture

Multiple Choice

50.0 g of rust (Fe₂O₃) was made when 34.97 g of iron (Fe) reacted with the oxygen in the air. What percent of rust is iron?

• 85.0%

• 69.9%

• 30.1%

• 15.1%

Multiple Choice

When 6.99 g of iron (Fe) reacted with the oxygen in the air, 10.0 g of rust (Fe₂O₃) was created. What percent of rust is iron?

• 85.0%

• 69.9%

• 30.1%

• 15.1%

Multiple Choice

You have calculated the percent of iron in two different rust samples.  Based on your answers, what type of matter is rust?

• element

• compound

• mixture

Multiple Choice

What is the mass percent of salt in sea water if there are 4.03 g in a 389.43 g sample?

• 99.1%

• 38.5%

• 4.03%

• 1.03%

Multiple Choice

What is the mass percent of each metal component found in solder when 910 g of tin is combined with 780 g of lead?

• 50% tin; 50% lead

• 91% tin; 78% lead

• 54% tin; 46% lead

• 13% tin; 26$ lead

Molar Mass

​The total mass of one mole of a compound is called the molar mass. This is found by adding the molar mass of each element together multiplied by the number of atoms of that element in each molecule.

​You will use a periodic table to find the mass of each element

Multiple Choice

What is the molar mass of the compound carbon dioxide, CO₂?

(According to a periodic table,

the molar mass of carbon is 12.01 g and the molar mass of oxygen is 16.00g)

• 28.01 g/mol

• 44.01 g/mol

• 40.02 g/mol

Multiple Choice

If the molar mass of carbon dioxide is 44.01 g, what is the mass percent of oxygen in the compound?

(According to a periodic table, the molar mass of oxygen is 16.00g)

• 36.36 %

• 27.29 %

• 72.71 %

Multiple Choice

According to the law of definite proportion, the mass percent of oxygen will always be the same in a carbon dioxide molecule.

• True

• False