What is the mass of 3.57 x 10^24 molecules of Ca(No3)2 ?

50.3 g

Explanation:

Start by converting the number of molecules of methanol to moles by using Avogadro's constant.

9.45

⋅

10

24

molecules CH

3

OH

⋅

Avogadro's constant

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1 mole CH

3

OH

6.022

⋅

10

23

molecules CH

3

OH

=

15.69 moles CH

3

OH

Now, in order to go from moles to grams, you need to use the molar mass of the compound, i.e. the mass of exactly  

1

mole of methanol.

Use the molar mass as a conversion factor to get

15.69

moles CH

3

OH

⋅

32.04 g

1

mole CH

3

OH

=

50.3 g

−−−−−  

The answer is rounded to three sig figs, the number of sig figs you have for the number of molecules present in the sample.

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