Mass Percent

Mass Percent is the percentage of a particular element within a compound. 

Atomic Mass Vs. Molecular Mass

Concept: Understanding the difference between Atomic Mass & Molecular Mass

Video Transcript

We're going to say the mass per mole of an element is also known as its atomic mass. Remember, that's just the mass of all the protons and neutrons found within an element. The reason we don't care about the number of electrons is because electrons weigh a lot less than protons and neutrons.
Now when it comes to the mass per mole of a compound, though, a compound is made up of more than one element combined. It could be the same element or it could be different elements combined together. We're going to say here we no longer call it atomic mass but instead we're going to call it molar or molecular mass.
Now whichever case you decide to look at, whether it be the atomic mass of an element or the molar mass of a compound, both have the same exact units. The units will be grams per mole. 

In order to find the atomic mass of an element you must look it up on your periodic table. 

Example: What is the total mass of each of the following elements?

a. Sodium                                              b. Gold                                                 c.  Mercury 


The total mass of a compound is the sum of each elemental atomic mass. 

Example: What is the total mass of each of the following compounds?

a. N2O5                                                            b. C12H22O11                                        c.  (NH4)3PO4


Examining Mass Percent

Concept: Understanding Mass Percent

Video Transcript

Now mass percent is just simply the mass of an element that's found within a given compound. Now the formula is just simply mass percent equals mass component divided by the total mass of the compound times 100.

Example: What is the percentage of carbon in sodium hydrogen carbonate, NaHCO3?


In order to calculate the molecular mass or weight of a compound you must look up the atomic mass or weight of each element. 

Example: A sample of toothpaste contains tin (II), SnF2. Analysis of a 5.25 g sample contains 8.77 x 10-3 g of F. What is the percentage of tin (II) fluoride in the sample? 


To go from the atomic mass of an element to the molecular mass of a compound you must use stoichiometric relationships. 

Problem: Hemoglobin contains 0.33% iron and has a molecular weight of 68 kg. How many iron atoms are in each molecule of hemoglobin?


Mass Percent Additional Practice Problems

What is the % of carbon by mass in Calcium Cyanide Ca(CN) 2?

A. 13.039 %

B. 17.541 %

C. 26.078 %

D. 30.412 %

E. None of these choices are correct.

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Suppose you are working for a pharmaceutical company, and your boss has tasked you with identifying the contents of a bottle containing an unknown white crystalline compound. You have narrowed down the possibilities to sodium carbonate (Na2CO3) and sodium oxalate (Na2C2O4). Sodium carbonate is relatively harmless (a common ingredient in commercial water softeners), but sodium oxalate is extremely toxic to humans; it is therefore very important that you properly identify the unknown compound. You perform an experiment, and your results show the compound is about 43% sodium by mass. Use this information to identify your compound.

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Calculate the mass percent composition of sulfur in Al  2(SO4)3.

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The main source of copper on earth is a mineral called chalcocite, which is copper(I) sulfide.

a)  What is the formula of this compound?  _________

b)  What is the mass percent of copper in this compound?

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What percent of the mass of the compound, K2PbO3, is due to lead?

A. 83.1%

B. 88.8%

C. 95.0%

D. 62.1%

E. 56.9%

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A concentrated solution of glucose (C6H12O6) in water 75.3 % glucose by mass. Calculate the mole fraction and the molarity of glucose solution. (The density of the solution is 1.03g/mL)

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What is the percent mass of H2O in Epsom salt, CaSO4·7H2O?

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Phenylenediamine (pictured below) is mainly used as a component of engineering polymers and composites. It is also an ingredient in hair dyes and is occasionally used as a substitute for henna.

What is the percent mass of carbon in phenylenediamine?

a. 66.6%

b. 11.1%

c. 7.45%

d. 25.9%

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Calculate the mass percent composition of sulfur in Al 2(SO4)3. The molar mass of Al2(SO4)3 is 342.17 g/mol.

A) 9.372%            

B) 21.38%      

C) 28.12%      

D) 35.97%      

E) 42.73%

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What mass of NaCl is contained in 24.88 g of a 15.00% by mass solution of NaCl in water?

A) 20.00 g

B) 21.15 g

C) 3.73 g

D) 1.50 g

E) 12.44 g

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Calculate the mass percent composition of lithium in Li 3PO4.

A) 20.82 %

B) 30.72 %

C) 55.27 %

D) 17.98 %

E) 26.75 %

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The molecular formula for a sugar molecule is C12H22O11. What is the percent of carbon in the molecule?

A) 42.12%

B) 2.37%

C) 26.67%

D) 67.88%

E) 8.34%

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The complete combustion of 0.285 g of a pure hydrocarbon yielded 0.878 g CO 2 and 0.405 g H2O. What is the mass % C in this hydrocarbon?

A) 24.0%

B) 72.7%

C) 27.3%

D) 32.4%

E) 84.1%

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Calculate the percentage by mass of lead in Pb(NO3)2

a. 38.6%

b. 44.5%

c. 62.6%

d. 65.3%

e. 71.2%

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Malachite is a green colored mineral that is 57.5% copper.  What mass of copper is present in a 250.0 g sample of malachite?               

A)  63.6 g            

B)  127 g             

C)  435 g             

D)  36.5 g            

E)  144 g

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Calculate the mass percent composition of sulfur in Al 2(SO4)3.

A) 28.12 %
B) 9.372 %
C) 42.73 %
D) 21.38 %
E) 35.97 %

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What is the weight percent, with respect to RbCl, of a solution if you mix 20.452 g of RbCl in 275.00 mL of water 31oC? The density of water is 0.9954 g/cmat 31oC. 

A. 6.95%
B. 7.47%
C. 6.89%
D. 7.40%
E. 7.44%

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Clonidine, C9H9Cl2N3 is a antihypertensive drug often used to treat attention-deficit hyperactivity disorder. What is the percent composition of nitrogen in Clonidine?

A. 6.09%
B. 46.97%
C. 13.04%
D. 30.81%
E. 18.27% 

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The compound dibromobis(ethylenediamine)cobalt(III) chloride has a formula of [Co(C2N2H8)2Br2]Cl. What is the mass percent hydrogen in the compound?

a. 0.27%

b. 4.32%

c. 2.16%

d. 8.63%

e. 0.54%

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What is the mass percent of hydrogen in LiNH 2?

A. 4.39%

B. 50.0%

C. 11.4%

D. 8.78%

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Calculate the mass percent composition of lithium in Li3PO4.

A) 26.75 %

B) 17.98 %

C) 30.72 %

D) 55.27 %

E) 20.82 %

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A 1.28 g sample of copper is heated with sulfur to produce 1.6 g copper sulfide. Calculate the mass percentages of copper and sulfur in the compound.

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Manganese comprises 1.3 x 10 -1 percent by mass of the elements found in a normal healthy body. How many grams of manganese would be found in the body of a person weighing 183 lb?

A. 87.6 g

B. 45.6 g

C. 879 g

D. 112 g

E. 108 g

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What is the percentage by mass of N in Cu(NO 3)2?


a) 4.38

b) 8.75

c) 14.9

d) 33.9

e) 70.0

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A compound of P and O is 56.34% P by weight. What is the simplest formula of this compound?

a. PO2

b. P2O7

c. P4O5

d. P2O3

e. P3O2

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A sample is composed of a mixture of NaBr and CuSO4.5H2O. A chemist decided to analyze the mixture by dehydrating it by heating it to 120°C for 12 hours which removed all of the water present. If a 6.523 sample weighed 4.967 g after dehydration, what is the percentage of NaBr in the original hydrated sample?


a) 22.4

b) 33.9

c) 39.2

d) 79.7

e) none of the answers is within 5% of the correct answer.

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A crystalline sample that has the formula MgCl 2.6H2O loses a portion of its water when it is dried in an oven for several hours but undergoes no other changes. If the student, Dora the Explorer, started with 10.000 g of the compound and ended up with a residue that weighed 7.342 g., what is the formula of the residue?


a) MgCl2.H2O

b) MgCl2.2H2O

c) MgCl2.3H2O

d) MgCl2.4H2O

e) MgCl2.5H2O

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What mass of NaCl is contained in 24.88 g of a 15.00% by mass solution of NaCl in water?

  1. 20.00 g
  2. 21.15 g
  3. 3.73 g
  4. 1.50 g
  5. 12.44 g
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