### E.27

Posted:

**Thu Apr 05, 2018 10:12 am**Im having trouble with part b of the question. I found the grams in the H2O molecule to be 2.992x10^-23 but don't know how to approach finding the molecule number.

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=26&t=30420

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Posted: **Thu Apr 05, 2018 10:12 am**

Im having trouble with part b of the question. I found the grams in the H2O molecule to be 2.992x10^-23 but don't know how to approach finding the molecule number.

Posted: **Thu Apr 05, 2018 10:32 am**

1: convert 1kg to 1000 g

2: you'll need to convert first to moles

(value given of H20 in grams * 1 Mol H20) / (atomic mass of H20g) atomic mass found by using periodic table

3: Now we have the number of H20 moles so we can use Avogadro's Number (molecules/moles) to convert moles to molecules by multiplying it - causes the moles to cancel out and leaving us with our value in molecules.

Hope this helps

2: you'll need to convert first to moles

(value given of H20 in grams * 1 Mol H20) / (atomic mass of H20g) atomic mass found by using periodic table

3: Now we have the number of H20 moles so we can use Avogadro's Number (molecules/moles) to convert moles to molecules by multiplying it - causes the moles to cancel out and leaving us with our value in molecules.

Hope this helps

Posted: **Thu Apr 05, 2018 11:29 am**

Basically for part b, divide 1000g by the molar mass of H2O which is 18.02 g X mol^-1. Then multiply that by Avogadro's number, 6.022x10^23 moles X mol^-1. And that should get you your answer.

This is how it looks like:

(1000g/18.02 g X mol^-1)(6.022x10^23 moles X mol^-1) = 3.34 x 10^25 moles

This is how it looks like:

(1000g/18.02 g X mol^-1)(6.022x10^23 moles X mol^-1) = 3.34 x 10^25 moles