[Ilikecats12344]

Hello all, I need some help. My teacher has told me my calculations are wrong for the following, I will underline the numbers which are wrong. Could anyone point me in the correct direction?

a) Calculate the number of moles present for the solid reactant
Zinc (Zn) – Total weight/Atomic weight (65) – 3.06/65 = 0.05 mol-1
Sodium Carbonate (Na2CO3) – Na x 2 = 46 + C = 12 + O x 3 = 48 = 101
3.006/101 = 0.03 mol-1
Sodium chloride (NaCl) – Na = 23 + Cl = 35 = 58
3.024/58 = 0.05 mol-1
b) Show the enthalpy change of the reaction per mole of solid reactant.
Enthalpy change of the reaction per mole of solid reactant =
Zinc = 3.24/0.05 = -64.8 kJ/mol-1
Sodium Carbonate = 0.63/0.03 = -21 kJ/mol-1
Sodium Chloride = 0/0.05 = 0 kJ/mol-1

[mikasaur]

This is an incomplete question. What's the reaction going on here? The numbers you've underlined seem to be weights that may have been given elsewhere in the question prompt. We can't help you with this if you don't give us the entire prompt.

[Ilikecats12344]

Sorry the mass volumes are what I weighed out for each substance? If they're what my group got I don't understand why my tutors said its wrong. For example the mass of zinc weighed out needed to be 3g, we weighed out 3.06g. Is this maybe why he said it's wrong? It needs to be exactly 3 or is there something else? We couldn't get it to balance to 3 exactly because the scales were so sensitive.

[mikasaur]

So you measured out 3.06 g of Zn, 3.006 g of Na₂CO₃, and 3.024 g of NaCl?

Maybe it's a significant figures thing?

[Ilikecats12344]

Yeah we measured out 3.06g of Zn and so on for the rest. It's a re submission so I can't get this one wrong, so maybe it is a sf thing but should I be safe and just use the exact amounts specified on the method?

[mikasaur]

That seems like it would be less safe. Unless you're trying to figure out a theoretical value.

If you have a balance that measures out to .0001 g then you need to go out to that many sig figs. I feel like that's unlikely though.

Can you not ask your teacher why the values are wrong and work with him or her to correct them? We have little to go on here.

[Ilikecats12344]

I agree, I can't do this because it was due today and it was my day off today, so I spent the day working on this, aside from this, my tutor isn't allowed to give us help that would give away the answer which would mean nothing to me...

Otherwise, do my calculations for finding the number of moles present for the solid reactant i.e Zn look correct forgetting that the mass is wrong?

[Ilikecats12344]

would it help at all if I gave you the method? it contains stuff like the initial and final temp, mass of weighing boat before and after etc and the mass of zinc weighed out

[mikasaur]

Your method for calculating the moles of each of those compounds look correct. I would double check your calculation for sodium carbonate, though.

You can give the method. Let's see what we can do.

[Ilikecats12344]

I'll look over sodium carbonate, here is the method for the displacement of copper from copper (II) sulphate by zinc:

Using the pipette and filler, transfer 25ml of copper sulphate into a polystyrene cup.

Place the polystyrene cup in a beaker for extra support.

Using a thermometer, measure the temp of the copper sulphate. Record this in the space provided (below)

Tare a balance. Place the weighing boat on the balance and record the mass in space provided. Tare balance again and weigh out 3g of zinc. Record the mass of zinc weighed out

Empty weighing boat into cup

Stir mixture with thermometer until the temperate does not change further. Record this temp.

Find and record the mas of the empty weighing boat

Results

Initial temp of copper - 26 degrees
Final temp - 57 degrees
Mass of weighing boat before measuring out zinc - 2.16g
Mass of Zinc weighed out - 3.06
Mass of weighing boat after measuring - 0.022.

Im beginning to think it might be a sig fig problem as well considering two of my measurements have more decimal places/sig figs. Not sure though

[mikasaur]

Is 3.06 g the weight of the Zn or the weight of the Zn and the boat?

[Ilikecats12344]

The weight of the zinc as the balance was tared before putting on the zinc after putting the boat on

[mikasaur]

So...

• The balance read 0.000 g, you put the boat on
• The balance read something around 0.022 g, you tared it and it read 0.000 g again
• You put Zn in until it read 3.060 g

If that's the case, I don't know what the problem with your value is, besides sig figs. If you messed up the tare in some way then your problem is getting the actual weight of the Zn.

[mjc123]

I read it as the boat weighed 0.022g after weighing out the zinc and adding it to the reaction, i.e. there was a little zinc that stayed in the boat and didn't get added. So the mass of zinc that was added to the reaction (which is what you need) was actually 3.038 g.
As you've underlined all the weights, it looks as if you may have made this error consistently. Go back and check.