Can someone help me with finding out the Molar Mass and Percentage Composition for compounds?

Question

Here are two examples:

 

Mg₃P₂

 

&

 

Sr(OH)₂

 

 

I've been struggling with trying to find out the molar mass/percentage compositions for these questions but I just struggle to do so, if someone could do a step by step process on how to do this I would greatly appreciate it, thank you.

Answer 1

To determine the molar mass of a substance, use the periodic table and find the atomic mass average (atomic weight) of each element in the substance. Multiply each atomic mass by the number of atoms of that element in the formula. Finally, take the sum total of all elements. 

 

In Mg3P2, there are 3 magnesium atoms and 2 phosphorus atoms. Therefore, we use the periodic table to find the atomic mass of magnesium (24.3050g) and phosphorus (30.9737g), and multiply each by the number of time it appears in the compound:

Mg: 24.3050g x 3 = 72.915g

P: 30.9737 x 2 = 61.9474g

61.9474g + 72.915g = 134.8624 g/mol

 

To determine the percent composition by mass of any element in a compound, we take the mass of that element in the compound and divide it by the total mass of the compound, then multiply by 100 to convert the fraction to a percentage.

 

In a mole of Mg3P2, the mass of all of the magnesium atoms is (as found above) 72.915g and the total mass of the compound (as found above) is 134.8624g, so the percent magnesium by mass in this compound is as follows:

(72.915g/134.8624g) x 100% = 54% 

This means that magnesium accounts for 54% of the mass of Mg3P2. Since there are only two elements in the compound, phosphorus accounts for the other 46%:

(61.9474g/134.8624) x 100% = 46%

 

Do you think you could do the same thing for Sr(OH)2 following these same steps?