Calculate the number of moles of oxygen in 5.8 grams of Mg(OH)2 (Milk Of Magnesia).

Question

The answer I got was 0.099 mol

Answer 1

Hi there! You got the right answer!

MM of Mg(OH)2 = MM of Mg + 2 * MM of O + 2 * MM of H

= 24.3 g / mol Mg + 2 * 16.0 g / mol O + 2 * 1.01 g / mol H

= 24.3 + 32.0 + 2.02

= 58.32 g / mol Mg(OH)2

Mass Mg(OH)2 / MM Mg(OH)2 = Amount Mg(OH)2

m / MM = n

5.8 g Mg(OH)2 / 58.32 g/mol Mg(OH)2 = 0.099 mol Mg(OH)2

A substance's molar mass, MM, tells us how much mass that substance weighs per mole - a mole being just over half a million million million million particles of the stuff, or half a septillion of the stuff! As abstract and hard to imagine as that is - and hard to count, too - you can measure mass easily with a scale. Then, knowing how many moles you have is important for chemical reactions. You may know that 2 moles of A reacts with 1 mole of B and leaves no leftover A or B, while making 2 moles of C:

2A + B -> C

So then you would know how what you need to measure out to achieve what you need in the lab - or in the kitchen. After all, the measuring and combining parts of Chemistry are a lot like they are in cooking!

Hope this helps! Please reach out if you have more questions!