empirical+formulas

Here is an online empirical formula calculator.

Follow these steps to calculate empirical formula. These steps work whether you are given a percent composition or mass composition.
 * 1) List the elements and their respective percentages (or masses in grams).
 * 2) Divide each % or mass by the atomic mass of the element.
 * 3) Identify the smallest result from step 2, and divide each answer from step 2 by that value.
 * 4) If one or more results from step 3 has a .5 decimal, multiply ALL of the results in step 3 by 2.
 * 5) Round the results from step 3 (or 4) to the nearest whole number; these are the number of atoms of each element in the compound.

Example: determine the empirical formula for nitric acid, which has a mass composition of 1.60% H, 22.23% N, and 76.17% O.

H 1.60 / 1.008 = 1.59 / 1.59 = 1

N 22.23 / 14.01 = 1.59 / 1.59 = 1

O 76.17 / 16.00 = 4.76 / 1.59 = 3

The empirical formula for nitric acid is HNO 3.

Here is a practice worksheet:

Here is a good tutorial:

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**Lab Applications** The empirical formula for a hydrate can be calculated in a similar way. As a hydrate is heated, the water in its crystal structure will be driven off. By measuring the hydrated compound's weight, and its dry (anhydrous) weight after heating, the mass of water can be determined. Using the mass data, the percent by mass of the anhydrous compound can be calculated, as can the percent by mass of water. Here's an example. 12.00 grams of hydrated sodium acetate was heated sufficiently to drive off its water. The dry sodium acetate weighs 7.23 grams. To find the mass of water removed, subtract the dry mass from the hydrate mass: 12.00 g - 7.23 g = 4.77 g water. Now calculate each percent by mass: Here is where the empirical formula calculation comes in. First, divide each percentage by the formula weight of the substance: Then divide each result by the smallest number:
 * anhydrous salt: 7.23 g / 12.00 g = 60.3% sodium acetate
 * water: 4.77 g / 12.00 g = 39.8% water
 * sodium acetate (F.W. = 82 amu): 60.3 / 82 = 0.7354
 * water (F.W. = 18 amu): 39.8 / 18 = 2.211
 * sodium acetate: 0.7354 / 0.7354 = 1.00 NaC 2 H 3 O 2


 * water: 2.211 / 0.7354 = 3.00 H 2 O

This provides the hydrate formula: sodium acetate trihydrate, NaC 2 H 3 O 2 **·**3 H 2 O