[Evaldas]

Hi!
I bought myself a chemistry workbook to have more work since I like chemistry, and I'm having some difficulties, so I'd like to talk about those exercises here, if you don't mind.
First of all, it's the first time I see a positive ion Li⁺ and negative ions HCO₃^- and H₂PO₄^-, since none of the solubility tables I have include them...
So, what compounds, solutions Li⁺ makes with those two negative ions? What compounds with those two negative ions make positive ions H⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Al³⁺
Please help me  , I don't think that for example Li⁺ + HCO_3-  LiHCO₃ or am I wrong?

[Borek]

Not sure what you are asking about - Li⁺ is in many aspects similar to Na⁺, K⁺  and so on - after all, it is just a cation of alkali metal. Almost all salts are made of metal cation and acid anion.

Almost all salts, as protonated amines are not metals.

[Evaldas]

Well then what about for example:
H⁺ + H₂PO₄^-  H₃PO₄? Is this right?

[Borek]

This reaction is usually written as

H₃PO₄ H⁺ + H₂PO₄^-

and is a first step of the phosphoric acid dissociation.

[Evaldas]

Ok. Thank you . But here's another, it says above "more difficult exercises" and it certainly is    :
How many molecules of acetic acid there are in one mililiter of its 80% solution (ρ=1.07 g/cm³)? Consider that acetic acid's ionization is very slight.
First of all, what does the last sentence mean? What does it change [slight ionization]?

Here's what I did:
What's given: ρ=1.07 g/cm³
V_(solution's)=1 ml = 0.001 l
V_(CH3COOH)=0.8 ml=0.0008 l (since it's 80% solution)
What to find: N_(CH3COOH) - ?
Solution: m_(CH3COOH)=ρ x V = 1.07 g/cm³ x 0.8 ml = 0.856 g.
M_(CH3COOH)=12 + 3x1 + 12 + 16 + 16 + 1 = 60 g/mol
n_(CH3COOH)=0.856 g / 60 g/mol = 0.014 mol
N_(CH3COOH)= n x N_A = 0.014 x 6.02 x 10²³ mol^-1=0.08428 x 10²³ = 8.428 x 10²⁵ molecules.

My answer does not match with the answer given in the workbook, the answer there is 8.59 x 10²¹ molecules.

What is my mistake?

[cliverlong]

<< snip >>

=0.08428 x 10²³ = 8.428 x 10²⁵ molecules.

My answer does not match with the answer given in the workbook, the answer there is 8.59 x 10²¹ molecules.

What is my mistake?

One error is here

0.08428 x 10²³ = 8.428 x 10^-2 x 10²³ = 8.428 x 10²¹

which is a lot closer to the given solution.

I haven't studied the rest of your answer in detail but are you sure you have handled the 80% correctly and performed the other calculations accurately ? (that's a guess as to where the other problem may lie)

Clive

[Borek]

V_(CH3COOH)=0.8 ml=0.0008 l (since it's 80% solution)

That would be correct for volume/volume percentage; this is most likely weight/weight, so you should calculate mass of the solution first.

[Evaldas]

Ok,thank you . I got the right answer now.
Here's another, I'm COMPLETELY lost at this one, where do I even start?
After heating 4.92 grams of magnesium sulfate crystalohydrate 2.4 g of dry compound had been obtained. What is the formula of this crystalohydrate?
Now, again, I know that the answer is MgSO₄ x 7H₂O. But how do I get this answer?
Should it be something like this:
What's given: m_{(MgSO4 x xH2O)} = 4.92 g
m_(MgSO4)= 2.4 g
What to find: x - ?
Am I even close?

[Evaldas]

Ok, I think I solved it, could you please check it?
Here's what I did:
After heating 4.92 grams of magnesium sulfate crystalohydrate 2.4 g of dry compound had been obtained. What is the formula of this crystalohydrate?
What's given: m_{(MgSO4 ∙ xH2O)} = 4.92 g
m_(MgSO4) = 2.4 g
m_(xH2O) = 4.92 g - 2.4 g = 2.52 g
What to find: xH₂O - ?
Solution: M_(MgSO4) = 120 g/mol
M_(H2O)=18 g/mol
n_(MgSO4) = 2.4 g / 120 g/mol = 0.02 mol
n_(H2O) = 2.52 g / 18 g/mol = 0.14 mol
n_(MgSO4) : n_(H2O) = 0.02 mol : 0.14 mol = 1 : 7
Answer: MgSO₄ ∙ 7H₂O

[Borek]

Yep.

[Evaldas]

Hey, I need help again:
Here's the exercise, it says:
Under each of the models write the symbol of the atom or ion.

So first I assumed that darker dots are protons, and the ones with pluses are electrons, so this way I'd just count the number of protons and look in the periodic table at the atomic number since it's the same as proton number, and just find out the symbol and then see how many eletrons it lost or gained and write the symbol of ion. But the third one in the first line doesn't even have those dots that I assumed were protons. So what is what?

[Borek]

Looks stupid to me. Those with + are most likely protons, but it also looks like they used identical symbol for neutrons (mixed with protons) and electrons (orbiting around).

[Evaldas]

So the answers would be: first - Be^2-, but the second would be He? It can't be, it can't have a charge since it's inert gas... right? The third - H. The fourth should be Li^2- but if I can remember correctly there's only Li⁺ ion? And the fifth B^2-? Doesn't make much sense in my mind... I may ask the teacher, she has the answers for these workbooks...

[Borek]

What are charges of protons, neutrons and electrons?

[DrCMS]

As Borek has said check the number of protons compared with electrons to get the correct charge for the final answers.