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urahonky
Registered User regular

Another Chemistry question for you smart people out there.

I've spent about an hour on this problem and can't figure out how it got the answer "61.5g" Can someone PLEASE point me in the right direction? The book is entirely worthless.

============

Part 2:

How many grams of HCl are formed from reaction of 3.56g of H2 with 8.98g of Cl2?

Reaction: H2 + Cl2 => HCl

It seems like a simple question but I can't think of anyway to do this. Do I convert H2 and Cl2 to moles? How would this help me at all?

=============

Part 3:

Give the quantum numbers for the outer most electron (by filling order) in Co

Co

n = 3

l = 2

ml = -2,-1,0,1,2

ms = +- 1/2

My question is how they arrived at n = 3 and l = 2. I did the electronic configuration for Co, and it's:

1s2 2s2 2p6 3s2 3p6 3d7 4s2

Does that make sense? How do they arrive at n = 3, l = 2?

================

Part 4:

This one is hard to type out. Basically I am trying to find the formal charge of an atom, with a given structure.

The formula I have is: # valence electrons - # lone pair electrons - # bonds

One structure has Sulfur with two pairs of lone electrons and two bonds. The options are:

a. 0 b. 1 c. -1 d. -2 e. 2

I said e, because Sulfur has 6 valence electrons, so I subtract 4 from that ( 2 bonds and 2 lone pair electrons) to get 2. But his answer key says: "ba". I assume this is a typo, but neither b or a make sense. Am I doing something wrong?

The structure looks something like this:

H ---- S(two dots above, and two dots below)

C ---(3 bars)---- N:

The reason why I'm confused is that even without the typo the rest of the answers for this part don't make any sense. Am I right or is he?

A scientist wants to make a solution of tribasic sodium phosphate, Na3PO4 , for a laboratory experiment. How many grams of Na3PO4 will be needed to produce 750mL of a solution that has a concentration of Na+ ions of 1.50 ?

I've spent about an hour on this problem and can't figure out how it got the answer "61.5g" Can someone PLEASE point me in the right direction? The book is entirely worthless.

============

Part 2:

How many grams of HCl are formed from reaction of 3.56g of H2 with 8.98g of Cl2?

Reaction: H2 + Cl2 => HCl

It seems like a simple question but I can't think of anyway to do this. Do I convert H2 and Cl2 to moles? How would this help me at all?

=============

Part 3:

Give the quantum numbers for the outer most electron (by filling order) in Co

Co

n = 3

l = 2

ml = -2,-1,0,1,2

ms = +- 1/2

My question is how they arrived at n = 3 and l = 2. I did the electronic configuration for Co, and it's:

1s2 2s2 2p6 3s2 3p6 3d7 4s2

Does that make sense? How do they arrive at n = 3, l = 2?

================

Part 4:

This one is hard to type out. Basically I am trying to find the formal charge of an atom, with a given structure.

The formula I have is: # valence electrons - # lone pair electrons - # bonds

One structure has Sulfur with two pairs of lone electrons and two bonds. The options are:

a. 0 b. 1 c. -1 d. -2 e. 2

I said e, because Sulfur has 6 valence electrons, so I subtract 4 from that ( 2 bonds and 2 lone pair electrons) to get 2. But his answer key says: "ba". I assume this is a typo, but neither b or a make sense. Am I doing something wrong?

The structure looks something like this:

H ---- S(two dots above, and two dots below)

C ---(3 bars)---- N:

The reason why I'm confused is that even without the typo the rest of the answers for this part don't make any sense. Am I right or is he?

0

## Posts

1. Find the amount of Na3PO4 needed (moles)

1a. Find target concentration of Na+ in 750 mL

x = 1.125 mol Na+

1b. Convert to Na3PO4

1.125 mol Na+ : x mol Na3PO4

x = 0.375 mol Na3PO4

2. Calculate molecular weight of Na3PO4

P = 31

O = 16, O4 = 64

Na3PO4 = 164 g/mol

3. Calculate amount of Na3PO4 needed (g)

OrogogusonAlso i didn't realize that you can just make M == mol like that.

urahonkyonIsoonThe formula tells you how many of each ion you'll end up with. Since it's Na3PO4, assuming it dissolves entirely you end up with 3 Na+ ions and 1 (PO4)--- ion for each molecule of Na3PO4 you start with.

Like, if the formula was X57Y21, you'd know that each molecule of X57Y21 would dissolve into 57 Xs and 21 Ys.

OrogogusonurahonkyonurahonkyonThis is a limiting reactant question. If you have an infinite amount of Cl2 and a little H2, then your H2 supply determines how much HCl you can make. So you figure out if you have more moles of H or of Cl and then calculate how many g of HCl that would be.

The reaction should be H2 + Cl2 -> 2 HCl, though.

OrogogusonurahonkyonFor this question, you first have to find out how many moles of HCl can be formed. And to find that, you need to find out how many moles of H and Cl you have. What they give you is the mass of the H and Cl, which is a measure of quantity, but worthless in terms of chemical reactions, so you need to convert.

Number of moles of H:

3.56 / 1.00794 = 3.532 moles

Number of moles of Cl:

8.98 / 35.453 = 0.25329 moles

Now, H and Cl combine to form HCl in a 1:1 ratio, so it seems that we have more H atoms and Cl atoms. You need one H for every Cl, so in this case, we can only form 0.25329 moles of HCl. The mass of HCl formed is therefore:

0.25329 x (1.00794 + 35.453) = 9.235g

IsoonurahonkyonIn this case, you're mistaking H2 for H. With 3.56g of H2, you have 1.78 moles of H2, but 3.56 moles of H, and it's important to make that distinction in your calculations.

The formula for the reaction is H2 + Cl2 => 2HCl. You could perform the calculations like you did, e.g. find out the number of moles of H2, and Cl2, then convert, but it's far easier to just find out how many moles of H and Cl you have, because that's your final product. And referring to the final product makes things easier when the product is something like Fe2O3, and you just find out how many 'blocks' of Fe2 and O3 you have to build the molecule with.

IsoonYou can look at either side, but it's important to note that if you look at the left side, you get 1.78 moles because the MW of H2 is 2. This means you have 1.78 moles of H2. However, for the purpose of this calculation, you don't really care how much H2 you have, what you care about is how much H you have.

IsoonReferring to your earlier calculations, you have calculated the number of boxes of burgers (e.g moles of H2) you have. But in this calculation, what's important isn't how many boxes you have, but how many burgers you have. So, you then see that you have 20 burgers, and 40 servings of chips. Now, one meal requires one burger and a serving of chips, so you can only make 20 set meals. And then it's a matter of finding out how much 20 set meals weigh.

IsoonurahonkyonurahonkyonurahonkyonYou're going after 3d7. The 3 in 3d7 is where n = 3 comes from, and the d is where l = 2 comes from -- s=1, p=2, d=3, f=4.

You don't figure for 4s2 because 4s comes before 3d in the electron filling order.

OrogogusonurahonkyonOrogoguson1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

Just make sure to draw arrows to remind you how they fill.

There should be an easy pattern!

Fuzzy Cumulonimbus CloudonOnly two more days of this class left! Tomorrow is the Chapter 6/7 exam, and Thursday is the final. This is going to be a terrible week, but it'll be worth it in the end.

urahonkyonHowever, I believe the correct answer is 0. If S has two loan PAIRS, that's 4 electrons right there. Plus the two bonds and you get 6. 6-6=0.

ceresonurahonkyon