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Titration question

markosheehan

Member
Jun 6, 2016
136
I am confused on a titration question.

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I understand most of it but I do not see why they do not multiply their answer by 10 at the end because it says 100 cm^3 of water was analysed but that formula gives answers in moles/L so to make it into a liter they should multiply by 10 but they do not ?
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
I am confused on a titration question.

I understand most of it but I do not see why they do not multiply their answer by 10 at the end because it says 100 cm^3 of water was analysed but that formula gives answers in moles/L so to make it into a liter they should multiply by 10 but they do not ?
The unit of $100\text{ cm}^3$ cancels against the unit of $5.7\text{ cm}^3$.
That is, we're left with the ratio between those 2 volumes, which is unit-less.
That leaves the unit of the concentration $M=0.02\text{ g/L}$ (I can't read the subscript), which is already in $\text{g/L}$.
 

markosheehan

Member
Jun 6, 2016
136
ok. thanks . i think i get it.

i am confused on this question though.

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here they divide their answer by 4 at the end even though both units are in cm^3

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i also do not understand this question. i understand that they multiply at the end by 5 as there was a dilution factor of 5 but I do not see why they do not divide by 10 as the vinegar was in a 100 cm^3 volumetric flask .


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markosheehan

Member
Jun 6, 2016
136
anyone?
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
ok. thanks . i think i get it.

i am confused on this question though.

here they divide their answer by 4 at the end even though both units are in cm^3
The given concentration is in $\text{g/L}$.
That is, the number of grams that would be in 1 liter.
Since we're talking about $250\text{ cm}^3$, which is a quarter of a liter, we need to divide by 4.

i also do not understand this question. i understand that they multiply at the end by 5 as there was a dilution factor of 5 but I do not see why they do not divide by 10 as the vinegar was in a 100 cm^3 volumetric flask .
Again, the concentration is given in $\text{g/L}$, which is the number of grams that 1 liter would have.
And it's not the number of grams in the original flask - that would be 1/10th.
 

markosheehan

Member
Jun 6, 2016
136
in the vinegar titration they ask for the the concentration of the ethanoic acid in the original vinegar so if you get your answer in grams per liter. so would be not have to take into this consideration by dividing by 10 to find the amount of grams in the original solution.
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
in the vinegar titration they ask for the the concentration of the ethanoic acid in the original vinegar so if you get your answer in grams per liter. so would be not have to take into this consideration by dividing by 10 to find the amount of grams in the original solution.
Only if they would have asked for the 'grams' (the mass).
But they're asking for the 'concentration'. For concentration the volume is irrelevant - it has been 'divided out'.
So whether we have 1 liter with 60 g/L, or we have 0.1 liter with 60 g/L, they have the same concentration of 60 g/L.
Of course there are 60 grams in that liter, and only 6 grams in the 0.1 liter.
 

markosheehan

Member
Jun 6, 2016
136
Thanks . Ok so only divide when you are dealing with grams of a substance in a solution to find out the number of grams in that amount of solution.