Stankovic123's chem q's

[zvezda]

Oh I see. Thanks Mao

[zvezda]

A student wishes to prepare 500ml of a standard solution of any base of concentration 0.2500M. Would it be better to prepare the solution using solid sodium hydroxide or anhydrous sodium carbonate?
Would it be the sodium carbonate as it has a higher molar mass, therefore the potential to obtain an inaccurate weighing of the substance is minimized?

[zvezda]

Also, if one was performing a titration by titrating a solution of sodium hydroxide with a standard solution of hydrochloric acid, they would rinse the conical flask with de-ionized water, rinse the burette with the sodium hydroxide and rinse the pipettes with the hydrochloric acid wouldn't they?
According to heinemann, you would rinse the pipettes with the sodium hydroxide and the burette with the standard solution of HCl?
Isn't the standard solution placed in the conical flask rather than the burette?

[lzxnl]

You can do either. Think about it. Where you place the standard solution doesn't actually matter. For argument's sake, let us assume that we have a 0.500 M solution of HCl and an unknown solution of NaOH that is really 1.00 M.

Let us assume that first, we place the HCl in the conical flask, 20.00 mL of it. We know that we have 20 mL*0.5 L = 10 mmol HCl, so we need 10 mmol NaOH, which if it was actually 1.00 M, we would need 10.00 mL of it.

Now let's reverse things. If we have 20.00 mL of NaOH in the conical flask and titrate this, we have 2 mmol NaOH needing 40.00 mL HCl. In either case, as all we have changed is what is titrating what, the concentrations of the respective solutions cannot change. Therefore, it does not matter what you place in what. The calculations will still work out. Just make sure that your rinsing follows accordingly.

[Mao]

A student wishes to prepare 500ml of a standard solution of any base of concentration 0.2500M. Would it be better to prepare the solution using solid sodium hydroxide or anhydrous sodium carbonate?
Would it be the sodium carbonate as it has a higher molar mass, therefore the potential to obtain an inaccurate weighing of the substance is minimized?

anhydrous sodium carbonate is generally preferred. It's a better primary standard, since sodium hydroxide (even solid) can absorb atmospheric CO2.

Also, if one was performing a titration by titrating a solution of sodium hydroxide with a standard solution of hydrochloric acid, they would rinse the conical flask with de-ionized water, rinse the burette with the sodium hydroxide and rinse the pipettes with the hydrochloric acid wouldn't they?
According to heinemann, you would rinse the pipettes with the sodium hydroxide and the burette with the standard solution of HCl?
Isn't the standard solution placed in the conical flask rather than the burette?

Ultimately it doesn't matter.
In practice, we don't put NaOH into the burette, because the burette is hard to clean, and NaOH is also hard to clean, the combination of those two is a nightmare for lab technicians. 
Another line of argument is the "never add anything to acid", which is a safety concern at high concentrations, but in the context of a titration it's unlikely to have any significance.

[zvezda]

Yeah fair call about not mattering. Thanks to the both of you

[zvezda]

Hey,
With an acid base reaction between, say, sodium hydrogencarbonate and hydrochloric acid, how do you actually get the carbon dioxide and water molecules as products? Going by the browsed-Lowry theory, wouldn't the hydronium ion just donate a proton to the hydrogen carbonate ion to form carbonic acid in some sort of equilibrium reaction (as in, the reaction is reversible)?

[Mao]

Yes. What you say is spot on.


and THEN:



that's where the CO2 comes from.

[zvezda]

Yes. What you say is spot on.


and THEN:



that's where the CO2 comes from.

Oh ok. But even still, when we write the equation of the overall reaction, there aren't any reversible arrows? How so if there are 2 reactions within this one reaction that are in equilibrium?

[zvezda]

One other thing as well,
In reference to back titrations, when you add the strong acid/base to the weak acid/base, you keep doing this until all weak acid/base molecules have reacted? This is what's implied in heinemann I think, but I just wanted to make sure this is correct because as far as I knew, weak acids/bases do not react to completion.

Edit: I probably should have continued reading. Ignore this

[lzxnl]

If we think about what happens in the majority of reactions that involve carbonic acid, the CO2 escapes. Therefore we no longer have a closed system. We never actually reach equilibrium because any CO2 formed escapes as opposed to reacting with water to form H2CO3. This is why we can safely write a one-way arrow. If we put a lid on the container and prevented the gas from escaping, we would really then have to write equilibrium arrows.

[zvezda]

hey,
with acid-base titration curves, why arent they linear?

[Mao]

hey,
with acid-base titration curves, why arent they linear?

because

Also,

The resulting equations that shows as a function of acid added is therefore highly non-linear. An actual mathematical model would be quite complicated.

[zvezda]

because

Also,

The resulting equations that shows as a function of acid added is therefore highly non-linear. An actual mathematical model would be quite complicated.

oh thats right of course. cheers mao

[Mao]

To give you a brief idea:



where , is the concentration of the HCl being added, is the original concentration of