Question

How do you set up Avogadro's number for certain equations?
For example, I have:

a. Find the number of atoms of hydrogen in 1.37 g of CH4.
I got:
moles of CH4 = 1.37g / 16.04g/mol^-1= 0.085 moles.
0.085 moles x 6.022 x 10^23 mol^-1 =
5.1187 x 10^22 atoms.
Do I round that answer to 3 sig fig? That's typically standard in chem, correct?
So, 5.119 x 10^22 atoms.
Is that correct?

b. Find the mass of 4.56 x 10^15 molecules of N2O4.
I have:
4.56 x 10^15 molecules N2O4 x (1 mol N2O4/ 6.022 * 10^23 molecules N2O4) =
7.57223513 x 10^-9 moles.
What do I round this to? Is it 7.572 x 10^-9, is that an appropriate answer? Is this the mass or am I doing something wrong?

c. Find the number of molecules of CH2O in 0.55 L of a 1.25 M CH2O solution.

What would i do in this case? I was able to kinda figure out the other two problems but not this one. Where would I start? How would I use avogadro's number?

Answer 1

0.55L * 1.25mole/L * 6.02*10^23 molecules/mole = 4.13875*10^23 molecules

Answer 2

oobleck did c for you. I'll look at the others. You have at least two problems; one with significant figures and the other with units.

a. Find the number of atoms of hydrogen in 1.37 g of CH4.
I got:
moles of CH4 = 1.37g / 16.04g/mol^-1= 0.085 moles.
<b><i> Note that the molar mass of CH4 is 16.04 g/mol or 16.04 g*mol^-1 but not what you wrote. Also, why did you drop a significant figure? You're allowed 3 so it should be 0.0854 moles.</b></i>
0.085 moles x 6.022 x 10^23 mol^-1 =
5.1187 x 10^22 atoms.
<b><i>0.085 mols CH4 x 6.022E23 molecules/mol = 5.1187E22 molecules of CH4. But the problem asks for atoms of hydrogen. So multiply by 4 to get that number. Yes you round that number to 3 sig figures but that isn't a standard figure in chemistry. In chemistry or any other math/science field you round to the correct number according to the rules.</b></i>
Do I round that answer to 3 sig fig? That's typically standard in chem, correct?
So, 5.119 x 10^22 atoms.
Is that correct?
<b><i> That number to 3 s.f. is 5.12E22 but that x 4 will give you the atoms of hydrogen in CH4.</b></i>

b. Find the mass of 4.56 x 10^15 molecules of N2O4.
I have:
4.56 x 10^15 molecules N2O4 x (1 mol N2O4/ 6.022 * 10^23 molecules N2O4) =
7.57223513 x 10^-9 moles.
What do I round this to? Is it 7.572 x 10^-9, is that an appropriate answer? Is this the mass or am I doing something wrong?
<b><i> I didn't run the numbers but assuming your calculations are right, then your number of 7.57223513E-9 would be rounded to 7.58E-9. That's 3 s.f. Why 3? Because the rule in multiplication is "you're allowed to use in the answer the smaller of the two numbers multiplied." That 3 from the 4.56. If you're multiplying more than two numbers it's the smallest number of any of the numbers. For example, 1.2 x 5.99 x 3.114 x 4.3321 = 96.967265. You're allowed 2 s.f. from the 1.2. That answer would rounded to 97.0 (to 3 s.f.) but you are allowed only two so 97 to 2 s.f.
As for the correct answer, no. The question asks for the mass and you have moles. You have the first step. Next step is
moles x molar mass = grams.
I suggest you read about s.f. The rules are different for addition/subtration.</b></i>