E 27

[Aashka Popat 1A]

This was the question: (a) Calculate the mass, in grams, of one water molecule. (b) Determine the number of H2O molecules in 1.00 kg of water.

I found the molar mass of H2O and got 18.02 g/mol, but I'm not sure what to do from there, can anyone help me? Thanks

[Jorja De Jesus 2C]

I think that you covert the 1.00 kg to grams and then you just divide that number of grams by 18.02 g.This is because if one molecule of water is 18.02g, you need to see how many of those would make up 1.00 kg or 1000g. So it would be 1000/18.02 equals approximately 55.49 molecules. So there would be 55.49 molecules in 1.00 kg of water.

[Charlene Datu 2E]

Ok so to answer part a, you want to start with the information they give you (1 molecule water) and you're trying to get to grams of H2O. To get there, you have to convert the 1 molecule of water to moles of water by dividing by Avogadro's number (6.022 x 10^23), since 1 mol = 6.022 x 10^23 molecules. Then use the calculated molar mass of water (18.02g/mol) and multiply to convert from moles to grams. My answer was 2.99 x 10^-23g H2O, but if I'm wrong, please correct me :)

(1 molecule H2O) x (1 mol H2O/(6.022 x 10^23 molecules H2O)) x (18.02g H2O/1mol H2O)

[Anne Tsai 1F]

For part b, 55.49 is the number of moles, not the number of molecules in 1000 g of water. To get the number of molecules, you have to multiply by 6.022 x 10^23 molecules per mole, which should equal 3.34 x 10^25 molecules.

[Ayushi2011]

For part b, I'd find the moles of a kilogram of water which is 1000g/(18.02g/mol) which is 55.49 moles. Since 1 mole has 6.022x10^23 molecules, 55.49 moles will have 55.49x(6.023x10^23)molecules which is 3.34x10^25 molecules.

[Maria Poblete 2C]

Ok so to answer part a, you want to start with the information they give you (1 molecule water) and you're trying to get to grams of H2O. To get there, you have to convert the 1 molecule of water to moles of water by dividing by Avogadro's number (6.022 x 10^23), since 1 mol = 6.022 x 10^23 molecules. Then use the calculated molar mass of water (18.02g/mol) and multiply to convert from moles to grams. My answer was 2.99 x 10^-23g H2O, but if I'm wrong, please correct me :)

(1 molecule H2O) x (1 mol H2O/(6.022 x 10^23 molecules H2O)) x (18.02g H2O/1mol H2O)

I also got this answer, so I'm pretty sure this is correct!

[Aman Sankineni 2L]

Yup, that's what I got too.