How do you determine the atomic weight percent of an element in a compound?

Answer 1

To determine the weight percent of the elements in a compound, you only need to two things: the formula of the compound and a Periodic Table. With that, just follow these three steps.

1) Determine the molecular weight (or molar mass) of the compound. For instructions on how to do that, see the Related Questions links to the left.

2) For each element in the compound, multiply the atomic weight of that element by the number of times it occurs in the compound. Do this for each element you want to know the weight percent for. See the example below for more about how to do this.

3) For each element, take the answer you got in Step 2, and divide it by the number you got in Step 1, and then multiply by 100%. In other words, take the weight of each element in the compound, and divide by the total weight, and then multiply by 100 to make it a percent (instead of a fraction).

--EXAMPLE: What is the percent by mass of carbon and hydrogen in C9H20?

--ANSWER: First, according to Step 1, we must find the molecular weight of the compound. To do that we multiply the atomic weight of each element by the number of times it occurs in the compound:

(9 * 12.011) + (20 * 1.008) = 128.259 grams per mole.

Now, let's look at carbon (C). According to the formula, there are 9 carbons in the molecule. Multiplying them as in Step 2, we have:

9 * 12.011 = 108.099

Finally in Step 3, we take this and divide it by the total weight:

108.099 ÷ 128.259 = 0.84281

Then multiply by 100%:

0.84281 * 100% = 84.281% carbon by mass in C9H20.

Now we can do the same thing again, but for hydrogen (H):

20 * 1.008 = 20.16

20.16 ÷ 128.259 = 0.15718

0.15718 * 100% = 15.718% hydrogen by mass in C9H20.

Finally, we can do a quick double check. Since the compound only has carbon and hydrogen in it, the two percents that we found better add up to 100%!

84.281 + 15.718 = 99.999 .... which is just slightly off due to rounding errors, but that's close enough!

Answer 2

First lets assume you mean percent by mass which are the usual numbers given.

The determination of chemical formula from percentage by weight is achieved through the use of the concepts of molecular mass, empirical formula, and molar mass.

The molecular mass of a compound can be found by taking its constitute atoms and adding their relative Atomic Mass (a number given in atomic mass units (u), where a carbon 12 isotope is defined as having a mass of 12). This will give a compound's molecular mass for example - carbon dioxide is composed of one carbon atom, and two oxygen atoms; the relative atomic masses being to three significant figures 12.0 and 16.0 respectively to three significant figures; by adding 12.0 to 16.0 x 2 you find 48.0 the mass of a carbon dioxide molecule.

A compounds empirical formula is the chemical formula expressed in the simplest whole number ratio. For example take sucrose C12H22O11 the empirical formula of which being: C1H2O1.

The molar mass of a compound is a mass in grams equal to the relative atomic mass, that is if you take our carbon dioxide molecule above with a molecular mass of 48.0 its molar mass is 48.0grams.

Now, given a compounds percent composition and molecular mass you can find it's chemical formula. The first step in this process is finding the empirical formula.

To find the empirical formula given percentages, as a convenience image you had a 100 gram sample. In this imagined sample you need to take the masses of each element you would have (it's the same as the percent weight) and divide this by the molar mass to find moles. Next having done this find the simplest whole number ratio between moles of each element. This will give you the empirical formula.

Now having found the empirical formula we need to find the molecular mass of such a compound if it did exist. The molecular mass is found as above by multiplying each of the compound constitute parts by its relative atomic mass.

With the empirical formula its molecular mass, and the molecular mass of the actually compound in hand we can calculate the chemical formula. Take the molecular mass for the compound and divide this by the molecular mass of the empirical formula. This will give you a positive integer, or close enough to. Take this number and multiply each of the subscripts for the empirical formula by it. You are done.

For example - A you have determined through analytical analysis that the percent composition by mass of a compound is C-42%, H-6.4%, and O-51.6% you know the compound has a molecular mass of approximately 342u, what is the chemical formula?

First our 100g sample

-42 grams Carbon

-6.4 grams Hydrogen

-51.6 grams Oxygen

m(Carbon)/M(Carbon) = 42/12 = 3.5

m(Hydrogen)/M(Hydrogen) = 6.4/1 = 6.4

m(Oxygen)/M(Oxygen) = 51.6/16 = 3.225

C:H:O ≈ 1:2:1

Therefore the empirical formula of our compound is C1H2O1

M(C1H2O1) = 12.0 + 2 * 1.0 + 16.0 = 30.0u

M(compound)/M(C1H2O1) = 342 / 30.0 ≈ 11

Therefore our compound is: C1*11H2*11O1*11which gives C12H22O11

Answer 3

You need to know the chemical formula of this compound, the atomic weights of the elements contained in the molecule, calculate the molar mass and after this calculate the percentages.