Download Percent Composition Calculations: Finding the Percentage of Elements in Compounds and more Summaries Chemistry in PDF only on Docsity!
Percent Composition WS Name:__________________________________
Period________
Percent Composition: The percentage by mass of each element in a compound.
Example Calculate the mass of each element in potassium carbonate, K 2 CO 3_._
First calculate the molar mass for K 2 CO 3. Find the atomic mass of each element from the periodic table. Multiply it by the number of times it appears in the formula and add up the total.
2 Potassium atoms K 2 x 39.10g = 78.20g K 1 carbon atom C 1 x 12.01g = 12.01g C 3 Oxygen atoms O 3 x 16.00g = 48.00g O 138.21g K 2 CO 3
To find the percent of each element divide the part of the mass that pertains to that element with the total mass
Percent of Potassium
K =
78.20g K 138.21g K 2 CO 3 X 100^ =^ 56.58 %^ K
Percent of Carbon C =
12.01g C 138.21g K X 100^ =^ 8.69 %^ C 2 CO 3
Percent of Oxygen O =
48.00g O 138.21g K X 100^ =^ 34.73 %^ O 2 CO 3
Percent Composition Calculations Using the example above, calculate the percentage composition of each element in the formulas shown below. Show your work and circle your answers.
1. Magnesium carbonate MgCO 3 2. Sulfuric Acid H 2 SO 4 3. Sodium Nitrate NaNO 3 4. Ammonium Sulfate (NH 4 ) 2 SO 4
Hydrated Crystals: Crystalline compounds that retain water during evaporation are referred to as being hydrated or are said to contain water of hydration. The ratio of moles of water to moles of compound is a small whole number. The formula for the hydrated compound barium chloride is:
BaCl 2.^ 2H 2 O
The dot shows that for every one formula unit of BaCl 2 there are two molecules of water. The amount of water in a hydrate can be determined experimentally by heating the compound and driving the water off. The compound with the water removed is known as anhydrous.
5. Typical calculations are % water, % salt, and % of each ion. Find each of these for the above hydrated salt.
% H 2 O =
%Salt =
%Ba+2^ ions =
% Cl-1^ ions =
Use percent as a conversion factor Finally, %’s of each component of a compound can be used to calculate the mass of one piece within a certain sample of a compound. For example CaF 2 is approximately 50% Calcium ions. If you had 2 grams of calcium fluoride, about 1 gram would be calcium and about 1 gram would be fluoride ions since they each make up about ½ of the compound by mass. To show work we would multiply by the % of each ion by the total mass.
6. Exactly how many grams of fluorine are in 0.45 g of Calcium fluoride? Use % composition as a conversion factor. 7. How many grams of hydrogen are in 1.27 g of iron (III) hydroxide?
