# Question Video: Calculating the Number of Moles of Selenium From a Given Mass Chemistry

The recommended daily intake of selenium for an adult is approximately 0.068 mg. How many moles of selenium is this? Give your answer to two decimal places. [Se = 79 g/mol].

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### Video Transcript

The recommended daily intake of selenium for an adult is approximately 0.068 milligrams. How many moles of selenium is this? Give your answer to two decimal places. Selenium equals 79 grams per mole.

The mole is the SI unit of amount of substance. One mole of substance will contain 6.022 times 10 to the 23rd entities of that substance, where entities may be molecules, formula units, atoms, or particles. The molar mass is the mass of 6.022 times 10 to the 23rd entities or the mass of one mole of substance and is given the unit grams per mole. The amount of a substance in moles, the mass of a substance, and the molar mass of a substance can be related by the following key equation. π equals lowercase π divided by capital π, where π is the amount in moles, lowercase π is the mass in grams, and capital π is the molar mass in grams per mole.

Looking at the question, we wish to determine the amount of moles of selenium. We are given the molar mass in grams per mole and the mass in milligrams. However, according to our equation, the mass must be in grams. We will need to begin by converting milligrams into grams. There are 1000 milligrams in one gram. With this conversion factor in mind, we can convert the mass in milligrams into grams by multiplying by the conversion factor written as a fraction with milligrams in the denominator so that the unit milligrams will cancel. We perform the calculation and get a value of 6.8 times 10 to the negative fifth grams.

We can then substitute the mass in grams and the molar mass in grams per mole into the key equation. We perform the calculation and determine the number of moles to be 8.6076 times 10 to the negative seventh moles. However, the question asked us to report our answer to two decimal places. With appropriate rounding, we have determined that 0.068 milligrams of selenium is equal to 8.61 times 10 to the negative seventh moles.

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