Aim: How to calculate Percent Composition DO NOW: 1. What is the number of moles of potassium chloride present in 148 g? 2. What is the molar mass of.

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Aim: How to calculate Percent Composition

DO NOW:

1. What is the number of moles of potassium chloride present in 148 g?

2. What is the molar mass of CuSO4 5H2O?

3. What is the mass of 3.01 x 1023 atoms of carbon?

Percent CompositionThe percent composition is the mass

percentage of each type of atom (element) in a compound.

% mass of element = x 100%

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Percent Composition from Mass Data

Percent composition can be calculated from experimental data.

A student prepares a compound of tungsten chloride from 3.946 g of tungsten and 3.806 g of chlorine. Calculate the percent composition.

% W = x 100% = x 100% = 50.9 %% Cl = x 100% = x 100% = 49.1 %

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Percent Composition from Chemical Formula

% mass = x 100%

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Percent Composition from Chemical Formula

To calculate the percent composition of a component in a compound:

1. Find the molar mass of the compound by adding up the masses of each atom in the compound.

2. Calculate the molar mass of the component in the compound for which you are solving.

3. Divide the molar mass of the component by the total molar mass of the compound and multiply by 100

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Percent Composition from Chemical Formula

Step18C + 10H +4N + 2O(8 x 12.0 g/mol) + (10 x 1.0 g/mol) + ( 4 x 14.0 g/mol) + (2 x 16.0 g/mol) = 194.0 g/mol

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Step 2Carbon: x 100% = 49.5 %Hydrogen: x 100% =5.1 %Nitrogen: x 100% = 28.9 %Oxygen: 100% - (49.5 + 5.1 + 28.9) = 16.5 %

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Percent Composition of Crystal Hydrates

A HYDRATE is a CRYSTALLINE compound in which ions are attached to one or more WATER molecules.

Example: Na2CO3 10H2O

Notice how WATER molecules are BUILT INTO the chemical formula

Substances without water built into the formula are called ANHYDRATES.

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Problem: What is the percentage by mass of water in sodium carbonate crystals (Na2CO3 10H2O)?

Step 1: Calculate the formula mass for the hydrate.

Na = 2 x 23.0 g/ mol = 46. 0 g/mol

C = 1 x 12.0 g/mol = 12.0 g/mol

O = 3 x 16. 0 g/mol = 48.0 g/mol

H2O = 10 x 18.0 g/mol = 180 g/mol

Formula mass of hydrate = 286 g/mol

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Step 2: Set up and Calculate

% composition by mass = mass of part/ mass of whole X 100%

% H2O by mass =(180 g/mol)/(286 g/mol) x 100%

% H2O by mass = 62.9 %

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Percent Composition as a conversion Factor

http://wps.prenhall.com/wps/media/objects/477/488597/le0602.html

We can use mass percent composition as a conversion factor between grams of an element and grams of the compound.

The mass percent composition of sodium in sodium chloride is 39%.

This can be written as:

39 g of Sodium : 100 g of Sodium Chloride

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This can be written in fractional form:

or

These fractions are conversion factors between grams of Na and grams of NaCl.

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The FDA recommends that adults consume less than 2.4 g of sodium per day. How many grams of sodium chloride can you consume and still be within the FDA guidelines? Sodium chloride is 39% sodium by mass.

Known Unknown

2.4 g Na grams of NaCl

39 g Na : 100 g NaCl

2.4 g Na x = 6.2 g NaCl

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Practice Problems

1.A 45.9 g sample of copper, Cu, combined with oxygen to give 51.69 g of an oxide compound. The percent composition of this compound is:

a. 88.8 % Cu, 11.2% Ob.24.4% Cu, 75.6% Oc. 60.1% Cu, 39.9 % Od.11.3% Cu, 88.7% O

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2. Calculate the percent compost ion of each set:

H2O

H=___11.1%_____O=______88.9%______

H2O2

H = ____5.88%____ O =_______94.1%_____

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3. How many grams of sodium will combine with 567.0 g of Sulfur from Na2S? (Hint: use % composition as conversion factor)

815.6 g