# bloody trick question - TDS (1 Viewer)

#### Abtari

##### Member
hey guys,

this is prolly old news, but just wanted to clear this up. for that TDS question, they gave us data so that we could calculate the values of mass of dissolved solids and insoluble solids (achieved by evaporation and filtration resp). During the exam, i vaguely remembered adding up both soluble and insoluble solids and using that to find total dissolved solids...

dammit. was that a trick question? were we sposed to only use the mass of soluble solids?? nooooooooooooooooo

#### AreYouAlright?

##### Actuarial Co-op 2006
According to conquering chemistry; the amount of total dissolved solids can be measured first by filtering off suspended solids then by evaporating a sample of water to dryness and weighing the solids left.

I think that means it was only the stuff in the evaporating basin..

#### jimre

##### New Member
argh

Look, TDS refers to total DISSOLVED solids - not total DISSOLVED + UNDISSOLVED solids. So ur only meant to use the mass of the dissolved solids that were found in the water.

#### tigerchenyanhu

##### New Member
THAT's whai i did

i calculate the tds and the total undissolved solids and clarifeid them .Then i calulated the percentage of the tds of the total solids which can be calculted by simly together to get the percentage cause i think tds should be expressed as a percentage since the normal unit of it is ppm

#### HotShot

##### -_-
this pissed me off, do u include the filtered stuff + the evaporated shit? hope not
i just put evaporate shit/total *100 = TDS percenage.

##### Member
i think they had this similar q in the success one chemistry, where they separately worked out the undissolved solids and dissolved solids, got a percentage then somehow added them together. Sorry can't reli remember...

#### Abtari

##### Member
fuck...i think i just screwed that question up sooo bad...

does anyone know how many marks it was???

#### alia

##### New Member
Abtari said:
fuck...i think i just screwed that question up sooo bad...

does anyone know how many marks it was???
It was 2 marks and u had to put the conc in % w/v

#### Abtari

##### Member
im pretty sure u can't put percentage in terms of w/v..

altho u would have got the same answer, i think u had to change 500 ml of water to 500 grams of water, on the assumption that the density of water is 1g/cc..

#### alia

##### New Member
w/v?

Abtari said:
im pretty sure u can't put percentage in terms of w/v..

altho u would have got the same answer, i think u had to change 500 ml of water to 500 grams of water, on the assumption that the density of water is 1g/cc..
why cant you put in w/v? w/v is a % isnt it. I thought it was like g/100ml or something and u can put the answer in (example) 4 % w/v which is 4g/100ml

#### Abtari

##### Member
alia said:
why cant you put in w/v? w/v is a % isnt it. I thought it was like g/100ml or something and u can put the answer in (example) 4 % w/v which is 4g/100ml
w/v is a ratio..

technically, a percentage can only be taken by multiplying a ratio by 100, but the numerator and denominator of the ratio should be of the same units otherwise the dimension of the ratio is not 0..i.e. the units need to cancel out, in simple terms.. but i dont think this is critical - the answer remains the same

#### tigerchenyanhu

##### New Member
but

rememebr u can only assume that PURE water can have a density of 1g/cm^3! not a random polluted water or a sample obtained somewhere!

#### Abtari

##### Member
density of water is affected considerably by temperature or pressure...

the amount of pollutants has VERY LITTLE to do with the density of the water... if u throw in some sand and sodium chloride into a beaker of distilled water, then when u go to measure the density of that 'contaminated' water, u obviously wouldn't use that sample because it isn't pure water... u would do whatever is necessary (filtration, evaporation etc) to purify it, and then obtain density of water..

when i say water has a density of 1g/cc, it is assumed that we are talking about pure water...

#### alia

##### New Member
But % w/v is used for many solutions that contain a dissolved solid in a solution. Im not too sure now, cos i thought about that in the exam but now i am a bit confused