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Wize University Chemistry Textbook > Stoichiometry

Mass Percentages

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Mass Percentage

Shows the amount that each element in a molecule contributes to the overall molecular mass.

Wize Tip
To determine the mass percentage for a particular element, follow these steps:

1) Find the atomic mass contribution of the element in question
2) Find the molar mass of the molecule
3) Solve for the mass percentage of an element using the equation below (take the mass contribution of an element and divide by the total molar mass, then multiply by 100)

mass percentage of element=mass controbution of elementtotal mass of molecule×100\boxed{\displaystyle \text{mass percentage of element} =\frac{\text{mass controbution of element}}{\text{total mass of molecule}}\times100}


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Example: What is the percentage of oxygen by mass in sodium hydroxide, NaOH?


mass percentage of O=mass controbution of oxygentotal mass of NaOH×100\displaystyle \text{mass percentage of O} =\frac{\text{mass controbution of oxygen}}{\text{total mass of NaOH}}\times100

1) Mass contribution of oxygen:
One oxygen atom so mO=1×15.999 g/mol=15.999 g/molm_O = 1 \times 15.999\ g/mol = 15.999\ g/mol

2) Total mass of NaOH:mNaOH=(1×MMNa)+(1×MMO)+(1×MMH)=22.990 g/mol+15.999 g/mol+1.008 g/mol=39.997 g/molm_{NaOH}=(1\times MM_{Na})+(1\times MM_O)+(1\times MM_H)=22.990\ g/mol+15.999\ g/mol+ 1.008\ g/mol=39.997\ g/mol


3) Find mass percentage of oxygen:

mass percentage=15.999 g/mol39.997 g/mol×100=40.00%\textrm{mass percentage}=\dfrac{15.999\ g/mol}{39.997\ g/mol}\times 100=40.00\%


Therefore, oxygen contributes to
40
% of the overall mass of NaOH!

Example: Mass Percentages of Elements in A Molecule


Sodium Bicarbonate (NaHCO3) is a very common chemical frequently used in the food industry. Find the mass percentages of each element in sodium bicarbonate.

Molar masses of each element:
  • Na is 22.99 g/mol
  • H is 1.01 g/mol
  • C is 12.01 g/mol
  • O is 16.00 g/mol

The molar mass of NaHCO3 is: 22.99 + 1.01 + 12.01 + 3(16) = 84.01 g/mol

The mass percentage of each element:

mass percentage of element=mass controbution of elementtotal mass of molecule×100\displaystyle \text{mass percentage of element} =\frac{\text{mass controbution of element}}{\text{total mass of molecule}}\times100

mass % Na = 22.99 / 84.01 x 100 = 27.36 %
mass % H = 1.01 / 84.01 x 100 = 1.20 %
mass % C = 12.01 / 84.01 x 100 = 14.30 %
mass % O = 48.00 / 84.01 x 100 = 57.14 %

Practice: Finding the Mass Percentage of Oxygen in Ethanol

What is the mass percentage of oxygen in ethanol (C2H5OH)?
Extra Practice
An oxide of nitrogen contains 36.5% oxygen by mass. What is the empirical formula of the oxide?
Elements: Mass Percentage
A metal oxide with the formula M2O3 is 47.06 % oxygen by mass. What is the identity of the metal, "M"?
Mass Percentage II
A certain metal hydroxide with the formula M(OH)2 is 17.22 % oxygen by mass. What is the identity of the metal, "M" ?
Gas Stoichiometry
A 1.8 g mixture of KClO3 and KCl was heated until the complete decomposition of KClO3. The released O2 has a volume of 405 mL at 25 ⁰C and 745 Torr. What was the percent of KClO3 present in the mixture?
Elements: Mass Percentage
A metal oxide with the formula M2O3 is 21.2 % oxygen by mass. What is the identity of the metal, "M"?
Mass Percentage
A certain metal hydroxide with the formula M(OH)2 is 17.22 % oxygen by mass. What is the identity of the metal, "M" ?
Stoichiometry: Mass percentage
A 2.56 g sample of ZnSO4 ⋅ 7H2O is heated to dryness. Determine the anhydrous mass remaining after all the water has been driven off. (provide an answer with 2 decimal places and correct unit)
Stoichiometry: Mass percentage
Sodium Bicarbonate (NaHCO3) is a very common chemical frequently used in the food industry. Find the mass percentages of each element in sodium bicarbonate.
Round answers to the nearest percentage. If you think an answer is 60% for example, just enter 60 in the blank.
Chemical formula: Mass percentage
What is the mass percent of oxygen in Calcium carbonate, CaCO3\rm CaCO_3?
What is the mass percentage of copper in CuSO4?
Chemical Formula: Finding the Mass Percent of Oxygen in Ethanol
What is the mass percentage of oxygen in ethanol (C2H5OH) ?
Sodium Bicarbonate (NaHCO3) is a very common chemical frequently used in the food industry. Find the mass percentages of each element in sodium bicarbonate.