To convert an amount from milligrams to milliequivalents, convert the amount given into millimoles by calculating the atomic weight and dividing it by the amount of the substance in milligrams. Then, multiply the amount in millimoles by the valence number to get the amount in milliequivalents.

**Determine the atomic weight of the substance**The periodic table is used to determine the atomic weight of a given substance. For example, if you are asked to convert 300 milligrams of sulfate to milliequivalents, you would look at the atomic weight listed for each element and add the numbers together. The formula for sulfate is SO4^--, so the atomic weight is 32 + (16 * 4) = 96 grams per mole. Since the conversion rate for grams per mole to milligrams per millimole is 1:1, the atomic weight in milligrams per millimole is 96.

**Calculate the amount in millimoles**Divide the amount of the given substance by its atomic weight. For 300 milligrams of sulfate, the result is 300 milligrams divided by 96 milligrams per millimole equals 3.125 millimoles.

**Multiply the amount in millimoles by the valence number**If the substance being converted is a cation or anion, multiply its amount in millimoles by the number of charges to determine the amount in milliequivalents. If there is no charge on the substance, the amount in millimoles is the milliequivalent. Sulfate is an anion with a -2 charge, so the final equation is 3.125 millimoles * 2 = 6.25 milliequivalents.