Calculations from titrations -chemistry

[ana_mujahid]

Calculations of titrations

The standard calculation:
A simple question will look like this:

25.0cm3 of 0.100 mol dm-3 sodium hydroxide solution required 23.5 cm3 of dilute hydrochloric acid for neutralisation. Calculate the concentration of the hydrochloric acid.

NaOH (aq) + HCL (aq) --> NaCl (aq) + H2O (l)

You do titration to find the concentration of one solution, knowing the concentration of the other one.

Planning a route through the calculation

• Start with what you know most about. In this case, you know both the volume and the concentration of the sodium hydroxide solution.
Work out how many moles of this you have got
• Look at the equation to work out how many moles of hydrochloric acid that amount of sodium hydroxide reacts with
• Work out the concentration of the hydrochloric acid

Doing the calculation:

The experiment used 25.0cm3 of 0.100 mol dm-3 NaOH solution.

Number of moles of NaOH = (25.00 ÷ 1000) x 0.100 mol = 0.002 50 mol

The equation says that 1 mol NaOH reacts with 1mole HCL
Therefore 0.002 50 mol NaOH reacts with 0.002 50 mol HCL

That 0.002 50 mol HCL must have been in the 23.5 cm 3 of hydrochloric acid that was added during the titration – otherwise neutralisation wouldn’t have been occurred.

All you need to do now is find out how many moles there would be in 1000cm3 (1dm3) of this solution.

If 23.5cm3 contain 0.002 50 mol HCL
100cm3 contain (1000÷23.5) x 0.002 50 mol HCL = 0.106 mol
The concentration is therefore 0.106 mol dm-3.




okay my questions are why was the NaOH divided by 1000?

and how did he/she end up with: The equation says that 1 mol NaOH reacts with 1mole HCL
Therefore 0.002 50 mol NaOH reacts with 0.002 50 mol HCL
????

[Ibn Khattab]

you divide by 100 to convert from cm cubed to dm^3 or vice versa

[GAL-actic]

okay my questions are why was the NaOH divided by 1000?



it's in cm3 and it should be in dm3 so I guess that's why it's divided by 1000

[Mace]

I'll try to explain as best I can without writing on my whiteboard. :)

So basically looking at sodium hydroxide first:

You have a concentration of 0.100 mol per liter.

So given that you want to know how many mols are in 25.0cc of that solution.

So you take 25.0cc divided by 1000cc (one liter) and you get 0.025L of NaOH solution.

Then multiply by 0.100 mol per liter and you have 0.00250 mol of NaOH.


Now each single mol of NaOH reacts with a single mol of HCl. So 0.00250 mol of NaOH must react with 0.00250 mol of HCl.


Then given that you must have 0.00250 mol of HCl in a solution that is of volume 0.0235L, you then have a concentration of 0.106 mol/liter


Hope this helps.

[ana_mujahid]

oh i see, thanks, how about the other part of my question?

[GAL-actic]

NaOH (aq) + HCL (aq) --> NaCl (aq) + H2O (l)

It's 1:1-->1:1

so if you calculate the number of moles of one substance it's the same for all substances

[Joha]

it's in cm3 and it should be in dm3 so I guess that's why it's divided by 1000

yes, think of it this way. To go from a cm to a dm you would divide by 10. Right? A cm is a hundredth of a meter, a dm a tenth of a meter.

Now we're dealing with dm3 and cm3. So cube your scale factor. 10^3 = 1000

[GAL-actic]

I'll try to explain as best I can without writing on my whiteboard. :)

So basically looking at sodium hydroxide first:

You have a concentration of 0.100 mol per liter.

So given that you want to know how many mols are in 25.0cc of that solution.

So you take 25.0cc divided by 1000cc (one liter) and you get 0.025L of NaOH solution.

Then multiply by 0.100 mol per liter and you have 0.00250 mol of NaOH.


Now each single mol of NaOH reacts with a single mol of HCl. So 0.00250 mol of NaOH must react with 0.00250 mol of HCl.


Then given that you must have 0.00250 mol of HCl in a solution that is of volume 0.0235L, you then have a concentration of 0.106 mol/liter


Hope this helps.


You're good at explaining:up:

[ana_mujahid]

I'll try to explain as best I can without writing on my whiteboard. :)

So basically looking at sodium hydroxide first:

You have a concentration of 0.100 mol per liter.

So given that you want to know how many mols are in 25.0cc of that solution.

So you take 25.0cc divided by 1000cc (one liter) and you get 0.025L of NaOH solution.

Then multiply by 0.100 mol per liter and you have 0.00250 mol of NaOH.


Now each single mol of NaOH reacts with a single mol of HCl. So 0.00250 mol of NaOH must react with 0.00250 mol of HCl.


Then given that you must have 0.00250 mol of HCl in a solution that is of volume 0.0235L, you then have a concentration of 0.106 mol/liter


Hope this helps.


hmm, its not confussing anymore but its long, takes ages to get to the wanted answer

( Check ur PM, that the sort i want)

yep it helped alot :up:

[ana_mujahid]

oh yeah, number of moles in a solution = molarity x (volume of solution {cm^3}÷1000)

[ana_mujahid]

...

[Mace]

oh yeah, number of moles in a solution = molarity x (volume of solution {cm^3}÷1000)

Right.

Or, in other words, molarity x volume (in liters)

[ana_mujahid]

is there a formula to calculate what we just did?

[Mace]

is there a formula to calculate what we just did?

It's probably better just to reason each problem through. I don't know what your exams are like, but when I was in high school our teachers used to put trick questions in all the time to trip up people who were just memorizing equations and not really reasoning through the material.

But basically you're talking about something like:

(molarity of sol A)(vol of sol A) = (ratio of reagent A/reagent B)(molarity of sol B)(vol of sol B)

With molarity expressed as mol per unit volume. And volume in the same unit of volume as the molarity.

Ratio of reagent A to B is given in the chemical formula for the reaction.

This is just for this type of simple titration problem.

[ana_mujahid]

hmm i dnt think we use the same method as you in the USA, but its the same idea,

i got less then 6 hrs till school, im gona try to get come sleep.

thanks for your help today, i still need to practice more questions in titrations

good night/day

bye