[araneus]

First off, allow me to say thank god for this forum! I'm attempting to complete grade 11 University Preparation Chemistry via correspondence, but I've been out of high school for 17 years and I've always been terrible with numbers and math. I feel too old to be confused by something taught to teenagers, but I have never taken Chemistry before in my life so this is all very new to me! I desperately need to pass with a 70% minimum for college, but I feel utterly bewildered by all of these formulas. I'm sure you didn't need my life story, but there you have it! It's a pleasure to meet all of you and a blessing to find this place.

Anyhow, I'm stuck on the following question:

Calculate the molar mass of the following amount in order to determine which substance has the greater mass in the following situation:

4.02 x 10³⁰ atoms of sodium or 3.90 x 10³⁰ molecules of propane (C3H8):

So, I convert particles to moles and sodium has 6.68 x 10⁶ mol while propane has 6.48 x 10⁶ mol. Am I correct so far? Now, I think I need to convert the moles to grams, but I'm not sure I understand how. I think I need to multiply the atomic mass of sodium (22.99) and the atomic mass or propane (44.10) by Avogadro's number, but I'm not actually sure how to do that. The 10²³ part is confusing me. To make a long story short, I'm lost and I can't answer what is probably a very simple question.

Any time and help you can offer would be greatly appreciated. Thank you in advance.

[rabolisk]

Welcome to the forum. You've already used Avogadro's number in going from number of atoms/particles to moles. To go from moles to grams, you only need the molar mass (atomic weight), which is 22.99 g/mol for sodium and 44.10 g/mol for propane. Doing this is no different than going from hours to minutes (except that the conversion is 60 min/hour), or any other dimensional analysis. Hope that helps.

[opti384]

You won't need to use the Avogadro's number again. This might come in handy: the molar mass gives the mass of one mole of a substance.

[DrCMS]

You don't really need to do the maths to give the answer you can do it in your head which is bigger (4.02 x 23) or (3.9 x 44)?

[araneus]

Thank you all for your replies. I appreciate the time you are taking to help me. Please forgive me for still being a little confused here, but I'm really struggling with this course. I have been since I started. I just want to be sure that I understand this for when the exam comes.

I can see which is larger between (4.02 x 23) and (3.9 x 44). Propane (171.6) is larger than sodium (92.46), but is it that simple? Does the 10³⁰ not matter at all?

[DevaDevil]

well, according to the question, it specifically asked you to "calculate the molar mass"
so you should calculate it in my opinion.

DrCMS is correct in that the answer to the second part (which has the greater mass) is indeed that simple to reason.

[DrCMS]

well, according to the question, it specifically asked you to "calculate the molar mass"
so you should calculate it in my opinion.

The molar mass of sodium is 23 (22.99) and propane is 44 (44.1)

The question does not ask you to calculate the actual mass of the those amounts of sodium or propane.

[DevaDevil]

my bad, it asks indeed only the molar mass, not total mass, and the molar masses were indeed found.

still, calculating the masses is not that hard, but indeed not necessary for this question

[araneus]

I find it all hard!  Even calculating the masses. I have no idea how I will pass this exam if I'm having so much trouble half way through the course.

So do you think this answer would be acceptable?

Propane (171.6gNa) has a greater mass than sodium (92.46gC₃H₈).

The rest of the answers I worked on for this looked similar. This was the entire question (and my answers) as a whole:

27. Calculate the molar mass of each of the following amounts in order to determine which substance has the greater mass in each of the following situations:

a) 3.2 mol of aluminum or 4.6 mol of sulphur:
3.2 mol of aluminum (86.34gAl) has a greater mass than 4.6 mol of sulphur (47.48gS).

b) 3.1 mol of water or 0.1 mol of sugar molecules (C₁₂H₂₂O₁₁):
0.1 mol of sugar molecules (342.34gC₁₂H₂₂O₁₁) has a greater mass than 3.1 mol of water (18.02gH₂O).

c) 4.02 x 1030 atoms of sodium or 3.90 x 1030 molecules of propane (C3H8):
Propane (171.6gNa) has a greater mass than sodium (92.46gC₃H₈).

I'm not asking you to check all of my answers for me, just hoping that my answer to C, which you've helped me with here, fits in with the rest! Thanks again!

[DevaDevil]

So do you think this answer would be acceptable?

Propane (171.6gNa) has a greater mass than sodium (92.46gC₃H₈).

typo? you put the formulas with the wrong names

also, those are not grams you have there!
You can either answer it by using the "quick way", explaining the formula:
N (number of atoms) * M (molar mass) / A (avogadro's number) = m (mass in grams)
then explaining that, as Avogadro's number is equal in both cases, you can simply multiply number of molecules by molar mass to see which is the highest.

Or you can calculate the exact mass with the formula, and then show which is the larger.


for the other questions:
a) how many grams of sulfur? typo?
b) those are again not grams! You only calculated molar masses, now go to mass, using the amount of moles

[araneus]

This is getting really embarrassing  I feel completely Chemistry impaired here. I can't get any of these right no matter how hard I try!!

typo? you put the formulas with the wrong names

Gah, yes that was a typo.

also, those are not grams you have there!
You can either answer it by using the "quick way", explaining the formula:
N (number of atoms) * M (molar mass) / A (avogadro's number) = m (mass in grams)
then explaining that, as Avogadro's number is equal in both cases, you can simply multiply number of molecules by molar mass to see which is the highest.

I think that aside from doing it all wrong, I'm not understanding how it should be written.

for the other questions:
a) how many grams of sulfur? typo?
b) those are again not grams! You only calculated molar masses, now go to mass, using the amount of moles

Canadian spelling rather than proper chemical spelling on A. My bad. And I'm lost for B. It's all starting to look like gibberish to me. I'd quit while I'm ahead, but I can't because I need this to get into college. I suppose I'll go off and try to fix all of these and be back later to bug you kind people again. Thank you again for your time.

[DevaDevil]

what I meant with a) grams of sulfur is that if you assume sulfur as elemental S₁ (in steas of the also possible S₈), you get 4.6 mol * 32 g/mol = 147 grams, NOT 47!

and my advice for C is to calculate the number of grams of each compound. That way you will no longer get confused.

[araneus]

Based on your advice, I'm pretty sure I've fixed question A and B.

a) 3.2 mol of aluminum or 4.6 mol of sulfur:
3.2 mol of aluminum (86.34gAl) has a greater mass than 4.6 mol of sulfur (14.52gS).

b) 3.1 mol of water or 0.1 mol of sugar molecules (C₁₂H₂₂O₁₁):
3.1 mol of water (55.86gH₂O) has a greater mass than 0.1 mol of sugar molecules (34.23gC₁₂H₂₂O₁₁).

I'm still not getting anywhere with trying to convert C into mass though.
c) 4.02 x 10³⁰ atoms of sodium or 3.90 x 10³⁰ molecules of propane (C₃H₈):

You gave me the following formula, which is also in the course PDF:
N (number of atoms) * M (molar mass) / A (avogadro's number) = m (mass in grams)

So, here's where my very limited basic math skills fail. The exponents are confusing the crap out of me. I honestly have no idea what I'm supposed to do with them so you won't find them in my final answers!

Sodium:
N (4.02 x 10³⁰) * M (22.99) / (6.02 * 10²³) = m (152.76gNa)??

Propane:
N (3.90 x 10³⁰) * M (44.1) / (6.02 * 10²³) = m (28.55gC₃H₈)??

And this also can't be correct because the person above told me that propane has a greater mass.

 

[DrCMS]

Based on your advice, I'm pretty sure I've fixed question A and B.

a) 3.2 mol of aluminum or 4.6 mol of sulfur:
3.2 mol of aluminum (86.34gAl) has a greater mass than 4.6 mol of sulfur (14.52gS).

NO! you have already been told that 4.6mol of sulphur weighs 147.2g.  Check your maths and think more.  1 mole of sulphur weighs 32g so how can 4.6 moles be less than 1 mole???

b) 3.1 mol of water or 0.1 mol of sugar molecules (C₁₂H₂₂O₁₁):
3.1 mol of water (55.86gH₂O) has a greater mass than 0.1 mol of sugar molecules (34.23gC₁₂H₂₂O₁₁).

This one is correct.

I'm still not getting anywhere with trying to convert C into mass though.
c) 4.02 x 10³⁰ atoms of sodium or 3.90 x 10³⁰ molecules of propane (C₃H₈):

You gave me the following formula, which is also in the course PDF:
N (number of atoms) * M (molar mass) / A (avogadro's number) = m (mass in grams)

So, here's where my very limited basic math skills fail. The exponents are confusing the crap out of me. I honestly have no idea what I'm supposed to do with them so you won't find them in my final answers!

And that is why your answer is wrong.

These are quite simple things to understand and work with

1x10¹ = 10
1x10² = 100
1x10³ = 1000
1x10⁴ = 10000
1x10⁵ = 100000
1x10⁶ = 1000000
etc. etc.

4.5x10⁶ = 4500000
but 0.45x10⁶ should be written as 4.5x10⁵ = 450000
and 45x10⁶ should be written as 4.5x10⁷ = 45000000

now to do some maths with them
say 1000000/100 = 10000
in scientific notation this would be 1x10⁶ / 1x10² = 1x10⁴

6-2 = 4

1x10²⁰ / 1x10⁸ = 1x10¹²

20-8 = 12

and 1x10³⁰ / 1x10²³ = 1x10⁷

So for the sodium
 22.99*4.02x10³⁰ / 6.02x10²³ = (22.99*4.02/6.02)x10⁷ = 15.35x10⁷ which is the same as 1.54x10⁸g sodium

and for propane
44.1 * 3.9x10³⁰ / 6.02x10²³ = (44.1*4.02/6.02)x10⁷ = 28.57x10⁷ which is the same as 2.86x10⁸g propane

2.86x10⁸ is bigger than 1.54x10⁸ so the propane weighs more.

[araneus]

I've changed the answer to question A in the file that goes off for marking. 4.6 mol of sulfur (147.2gS) has a greater mass than 3.2 mol of aluminum (86.34gAl). I see where I went wrong now.

These are quite simple things to understand and work with

1x10¹ = 10
1x10² = 100
1x10³ = 1000
1x10⁴ = 10000
1x10⁵ = 100000
1x10⁶ = 1000000
etc. etc.

4.5x10⁶ = 4500000
but 0.45x10⁶ should be written as 4.5x10⁵ = 450000
and 45x10⁶ should be written as 4.5x10⁷ = 45000000

now to do some maths with them
say 1000000/100 = 10000
in scientific notation this would be 1x10⁶ / 1x10² = 1x10⁴

Thank you, this was very helpful. I have a much better understanding of it now. It wasn't explained in our PDF notes because I guess they assume that we should have some basic understanding of this math! I don't mean to frustrate anyone here, but I failed grade 10 math seventeen years ago, so even the simple calculations necessary for this Chemistry course look a bit alien to me. I would not even attempt to take this course if it were not absolutely necessary for the college program I've already applied to because clearly, it's going to continue to be a struggle for me.

2.86x10⁸ is bigger than 1.54x10⁸ so the propane weighs more.

Thank you again. Just so I'm absolutely clear, 2.86x10⁸g is equal to 286,000,000g but just not written that way, correct?