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Chemistry: Percent Composition

Percent Composition

Sometimes, it's handy to figure out how much of an element is present in a chemical compound. Let's say that you're trying to add extra calcium to your diet by taking a calcium supplement containing 1 gram of CaCO3. How much calcium are you actually getting?

To solve this problem, we need to figure out the percentage of calcium present in this compound. This percentage is referred to as either the mass percent or weight percent and is found by using the following formula:

  • Mass percent = mass of the element we're interested inmolar mass of the whole compound × 100%

Let's use this equation to find the amount of calcium in one gram of a CaCO3 supplement:

You've Got Problems

Problem 3: Determine the amount of silver present in 25 grams of silver nitrate.

The mass of calcium in this compound is equal to 40.1 grams because there's one atom of calcium present and calcium has an atomic mass of 40.1 amu. The molar mass of the compound is 100.1 grams. Using the handy equation above, we get:

  • Mass percent = 40.1 g Ca100.1 g CaCO3 × 100% = 40.1% Ca

Our result means that 40.1% of the mass in the calcium carbonate supplement is caused by calcium. To find the total mass of calcium in the supplement, we find 40.1% of one gram to get a final answer of 0.401 grams of calcium.

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Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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