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?
0.55L * 1.25mole/L * 6.02*10^23 molecules/mole = 4.13875*10^23 molecules
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.
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.
0.085 moles x 6.022 x 10^23 mol^-1 =
5.1187 x 10^22 atoms.
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.
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?
That number to 3 s.f. is 5.12E22 but that x 4 will give you the atoms of hydrogen in CH4.
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?
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.
To set up calculations using Avogadro's number, you need to have an understanding of moles, which is a unit used to measure the amount of a substance. Avogadro's number (6.022 x 10^23) represents the number of entities (atoms, molecules, ions) present in one mole of a substance.
Now let's go through each problem:
a. Find the number of atoms of hydrogen in 1.37 g of CH4.
First, you correctly calculated the number of moles of CH4. Then, to find the number of hydrogen atoms, you multiply the number of moles of CH4 by Avogadro's number. Your calculations are correct, and rounding to three significant figures is appropriate. Therefore, the answer is 5.119 x 10^22 atoms.
b. Find the mass of 4.56 x 10^15 molecules of N2O4.
You started correctly by setting up a conversion factor using Avogadro's number. Your calculation for moles is also correct. To find the mass, you need to multiply the number of moles by the molar mass of N2O4. The molar mass of N2O4 is 92.02 g/mol. Therefore, you need to multiply 7.57223513 x 10^-9 moles by 92.02 g/mol. Multiplying these values will give you the mass of N2O4 in grams.
c. Find the number of molecules of CH2O in 0.55 L of a 1.25 M CH2O solution.
In this case, you need to use the given concentration (1.25 M) to calculate the number of moles of CH2O. Molarity (M) is defined as moles of solute per liter of solution. So, you need to multiply the concentration by the volume (0.55 L) to find the number of moles.
Once you have the number of moles, you can use Avogadro's number to find the number of molecules. Multiply the number of moles by Avogadro's number to find the number of molecules of CH2O.
Remember to use the correct units and pay attention to any conversions required in each problem. Also, make sure to round your final answers appropriately based on the significant figures given or implied in the question.