SAT II Chemistry : Avogadro's Number

Study concepts, example questions & explanations for SAT II Chemistry

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Example Questions

Example Question #1 : Avogadro's Number

How many nitrate ions are in one mole of ?

Possible Answers:

Correct answer:

Explanation:

Note that there are two nitrate () ions for every formula unit of calcium nitrate. This means that in one mole of calcium nitrate, there are two moles of nitrate ions. To go from moles to ions, multiply 2 by Avogadro's number. 

Example Question #2 : Avogadro's Number

How many hydrogen atoms are in one mole of ?

Possible Answers:

Correct answer:

Explanation:

In one molecule of , there are 2 ammonium ions, each of which has 4 hydrogen atoms for a total of 8 hydrogen atoms. Thus, to find the number of hydrogen atoms in one mole of , one must multiply Avogadro's number by 8.

Example Question #3 : Avogadro's Number

Calculate the number of aluminum ions in  moles of .

Possible Answers:

Correct answer:

Explanation:

In order calculate the number of aluminum ions, we must first find the number of aluminum ions in the entire compound. In this case, there are two molecules of   for every molecule of . After understanding this, we can use Avogadro's constant to determine the number of atoms (more specifically ions) of aluminum. See equation below for specific calculations.

 

Example Question #4 : Avogadro's Number

How many atoms of chloride are in 0.2550 g of aluminum chloride, ?

Possible Answers:

Correct answer:

Explanation:

The formula for aluminum chloride is . The molar mass of  is 26.98 g and the molar mass of  is 35.45 g. There are three  atoms in . To calculate the molecular mass of , we need to find the sum of the mass of one aluminum atom and three chlorine atoms:

The total molecular weight is 130.33 g. Starting with the grams of , we convert to the amount of moles  using the molecular weight value we just calculated. Then we calculate the number of moles of chloride ion in . using the 3:1 ratio. Finally, we calculate the number of atoms by using Avogadro's number. See calculations below:

 

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