PERCENT COMPOSITION

The first step in finding percent composition is to find the molar mass of the compound that you are to prepare. Let's look at an example. Suppose you would like to prepare 500 grams of sodium chloride. To determine how much sodium and chloride you might need, you will first need to calculate the molar mass of the compound sodium chloride.

From these calculations, we see that one mole of sodium chloride has a mass of 58 grams. To determine the percentage of sodium and chloride present, we must compare each ingredient to the molar weight. Let's continue with our example.

From these calculations we can say that 60.4% of the 500 grams (or any amount) of sodium chloride will consist of the chloride ions or 60.4% x 500 grams = 302 grams. To check your work, add up the two percentages. They should equal 100% or very close to 100% (some small percentages may be lost due to rounding). 39.6% Na % Cl %

Let's take a look at another example. Suppose you had to prepare 700 grams of barium hydroxide. You will need to know how much of each ingredient will be required. Begin by finding the molar weight for barium hydroxide. Barium hydroxide Ba(OH ) 2 Barium: 1 x 137 grams/mole l37 grams Oxygen: 2 x 16 grams/mole 32 grams Hydrogen: 2 x 1 grams/mole = 2 grams Molar weight = 171 grams

(Note that a small percentage was lost due to rounding as we made our calculations.)

To complete this problem, take the percentage of each ingredient (barium, oxygen and hydrogen) of the total amount of the barium hydroxide desired. Ba: 80.1% of 700 grams = grams 0: 18.6% x 700 grams grams H: 1.2% of 700 grams = 8.4 grams We can say, therefore, that to make 700 grams of barium hydroxide, you must acquire grams of barium, grams of oxygen and 8.4 grams of hydrogen.

Find the percent composition of each compound below.

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YKY Empirical and Molecular Formulas

How do I know when I’ve reached the empirical formula for a compound? The answer is when you can no longer can reduce the ratio. Below which are empirical formulas.

Suppose you have received the following analysis for an unknown chemical compound you have just produced. The analysis for your compound is: 20% calcium 80% bromine Based upon the information received in the analysis, you can tell that you have a compound which contains calcium and bromine. To find the correct empirical formula for this compound you must first find the number of moles of each ingredient present and then reduce that ratio of moles to the nearest whole number ratio.

To begin finding the empirical formula for this unknown chemical compound, assume that you have 100 grams of the compound. (Assuming you have 100 grams of the compound makes the math calculation somewhat easier!)

20% of the 100 gram sample represents the calcium ingredient, which is the same as saying... 20% x 100 grams = grams of calcium present 0.20 x 100 grams = grams of calcium present 20 grams = grams of calcium present. Therefore, in our 100 gram sample, we have 20 grams of calcium and 80 grams of bromine. Recall that our goal is find the number of moles present of each ingredient. We now have the number of grams present of each ingredient, so we need to convert those grams of ingredients into moles of ingredients.

Grams x 1 mol = mol grams 20 grams Ca x 1 mol= 0.5 mole Ca 40 grams Ca 80 grams Br x 1 mol = 1.0 mole Br 80 grams Br Molar Mass

Summary 1. Assume you've been given 100 grams of the sample. 2. Convert the percentage of each element to grams. 3. Convert the gram amount of each element into moles. 4. Adjust the mole amount of each element to whole numbers by dividing by the lowest

Let's continue now with another example. Here is an analysis of an unknown compound that you have been asked to identify. The analysis for your compound is: 11.2% hydrogen 88.8% oxygen Work it out

we can see that we have a ratio of 11.2 moles of hydrogen for every 5.55 moles of oxygen. We can write the compound formula as follows.

Find the empirical formula for each compound.

Review