Answered By Faraz V

Calculate the molar mass of Mg(OH)2

Mg=24.3

H=1

O=16

So you have 58.32 gram/mol

Now divide 15 gram / 58.32 gram/mol

You end up with 0.26 mol

Answered By Kevin G

1) Make a list of what is given and what is needed:

Given:

15 grams_Mg(OH)2

Mg=24.30 g/mol

O = 16.00 g/mol

H = 1.01 g/mol

(via periodic table)

Need:

mols

2) Calculate the molar mass of Mg(OH)₂:

24.30 + 2*(1.01+16.00) = 58.32 grams_Mg(OH)2/mol 

3) Now we can use the formula:

 $n=\frac{m}{M}$n=mM  

Or you can think of it like this: 

$\frac{g}{\left(\frac{g}{mol}\right)}=\frac{\left(mol\times g\right)}{g}=mol$g(gmol ) =(mol×g)g =mol 

Either way, you get

 $\frac{15g}{\frac{58.32g}{mol}}=\text{0.2572}...\approx0.26$15g58.32gmol  =0.2572...≈0.26 

Answered By Chris W

So first you have to find out the molecular mass of Mg(OH)2. Once you have this mass, you can use unit analysis to find the number of moles!

Mg =24.3g/mol ; O x2 = 16 x 2 = 32g/mol; H = 1.01 * 2 = 2.02

When you add these you get 58.32g/mol

Too find moles -> moles = mass / molecular

moles = gram / (gram/mol)    **Notice how the gram units cancel out?**

moles = 15g / (58.32g/mol) 

moles = 0.2572... mol

number of moles of Mg(OH)2 = 0.26 mol

A really helpful trick with these kinds of conversions is to *watch your units* and make sure the proper ones cancel out (grams in this case) and that you are left with the right ones (mol in this case)!

Answered By David H

Set up the problem like this:

      $15g\times\frac{molMg\left(OH\right)2}{58.32g}=$15g×molMg(OH)258.32g =      

because 58.32g is the molecular weight of magnesium hydroxide (you can find this by adding up the weights of Mg+2(OH) from the periodic table and check your answer by googling "Mg(OH)2 molecular weight"). The grams cancel, and you get mols. 

Test your knowledge: How many mols in 23g of Calcium hydroxide (Ca(OH)2)?

(answer: 0.31 mols)