10g of water and sodium chloride?

Question

Compare the # of atoms present in 10.0 gram samples of water and sodium chloride

also, if you can/want,

Determine the mass of O2(g) required to completely combust with 10.0g CH4(g).
[2 and 4 are subscripts]

I am brand new to Chem this year and well, im not exactly sure what to do/look for
thanks for any help!
explanations are always nice, im really not here just to "get an answer and leave," i want to be able to understand it

Answer 1

The 10.0 grams is the same in both samples. The molecular masses (weights) are different. You need to find the number of moles in each sample. You do this by dividing the 10.0 grams by the MW.

for water H2O, the MW = 18.0153 g/mol
for Sodium chloride NaCl, the MW = 58.442 g/mol

One mole has 6.022 * 10^23 molecules.

10.0 g / 18.0153 g/mol = Number of moles of H2O
multiply this by 3 (atoms per molecule) then 6.022 * 10^23 to get Number of atoms.

10.0 / 58.442 g/mol = Number of moles of NaCl
multiply this by 2 (atoms per molecule) then 6.022 * 10^23 to get Number of atoms.

For the combustion, first write the balanced equation:
CH4 + 2 O2 --> CO2 + 2 H2O

one mole of Methane needs 2 moles of Oxygen gas.
10.0 g / 16.04 g/mol = 0.623 moles of Methane

0.623 * 2 = 1.246 moles of O2

each moles contains 32.00 g of Oxygen so

1.246 moles * 32.00 g/mol = # of grams of O2.

Answer 2

For the first question; water is 18g/mol, sodium chloride is 58.45 g/mol, so obviously there would be more atoms present in the 10g water sample. First, water weighs less per mole, than salt, so more molecules in a stated mass (10g). Also, each molecule of water has 3 atoms 2 H's and an O; each molecule of salt has only 2 atoms, a Na and a Cl. Note: the question said compare the numbers; not calculate them!

For the second question, generate a balanced equation:

CH4 + 2O2 >>>> CO2 + 2H2O

Now find out how many moles is 10g of CH4:

10g/[12 + (4 x 1)]g/mol = 0.625 mole of CH4

Now from the balanced equation we can find out moles of oxygen needed:

The ratio from the equation is 1 CH4 to 2 O2 (Remember O2 [oxygen gas] is 32 g/mol

So to get mass:

2 x 0.625mol x 32g/mol = 40g