Question #513f2

1 Answer
Sep 30, 2015

The answer is (a) #6.02 * 10^(23)#


So, you know that your sample of glass contains #"9 g"# of water.

Use water's molar mass to find how many moles of water you get in your glass sample.

#9color(red)(cancel(color(black)("g"))) * "1 mole water"/(18.02color(red)(cancel(color(black)("g")))) = "0.4994 moles water"#

A water molecule, #"H"_2"O"#, contains one oxygen atom and twp hydrogen atoms. This means that every mole of water, you have

  • one mole of oxygen
  • two moles of hydrogen

This means that the number of moles of hydrogen you have in that many moles of water will be

#0.4994color(red)(cancel(color(black)("moles water"))) * "2 moles H"/(1color(red)(cancel(color(black)("mole water")))) ~~ "1.00 moles H"#

Now, one mole of any element contains exactly #6.022 * 10^(23)# atoms of that element - this is known as Avogadro's number.

In your case, one mole of hydrogen will contain

#1.00color(red)(cancel(color(black)("mole H"))) * (6.022 * 10^(23)"atoms H")/(1color(red)(cancel(color(black)("mole H")))) = color(green)(6.02 * 10^(23)"atoms of H")#

SIDE NOTE The exact value will actually be #6 * 10^(23)# atoms of hydrogen, since you only gave one sig fig for the mass of water.