Q3 water experiments

1Q: name 2 species in swimming pool water known as free chlorine

1. hypochlorous acid (HOCl)

2. hypochlorite ion (OCL-)

1Q: free chlorine?

chlorine in the form of hypochlorous acid and hypochlorite ions

1Q: principle on which measurement of free chlorine present in water sample using a colorimeter is based?

absorbance directly proportional to concentration

1Q: name reagent suitable to test for free chlorine in a water sample

DPD tablet

1Q: how does this reagent indicate presence of free chlorine in a sample?

solution turns pink, the more free chlorine present, the more intense the colour

1Q: relationship between absorbance and concentration of free chlorine?

absorbance directly proportional to concentration so straight-line graph through origin will be obtained

1Q: give a reason why the concentration of free chlorine treated in drinking water is usually between 0.2ppm to 0.5ppm whereas in swimming pool water it should be between 1-5ppm?

swimming pool water has more bacteria

1Q procedure

1. 6 standard solutions of chlorine are made up: 0,1,2,3,4,5 ppm (note: 0ppm is just DI water!)

2. DPD tablet added so solutions turn pink (except 0 ppm bc contains no free chlorine)

3. solutions placed in colorimeter and an absorbance vs conc graph is plotted (straight line graph through origin bc directly proportional)

4. DPD added to a swimming pool sample and an absorbance reading is taken

5. graph is then used to determine concentration of free chlorine in pool water based on its absorbance (just draw a line from absorbance to graph and see corresponding conc!)

NOTE: when diluting effluent/slurry to get BOD and trying to find original concentration(/BOD if diluting twice)...

... first [O2] - second [O2] ppm is the "diluted" in the formula diluted x dilution factor = original!!!

also if diluting twice u just multiply by dilution factor twice

2Q: (i) concentration of total suspended solids in a water sample (ppm) procedure

1. given a water sample of known volume

2. weigh filter paper

3. filter water sample

4. dry filter paper in oven

5. reweigh filter paper

6. final mass - initial mass = g suspended solids (this is in the initial volume)

7. know and want with know: g -> given volume

want: mg -> 1000cm^3 to get ppm!!

2Q: (ii) concentration total dissolved solids in a filtered water sample (ppm) procedure

1. given volume (new one or use same as part (i)!!!)

2. weigh clean, dry conical flask

3. pour water in and use hot plate to boil off water

4. reweigh beaker

5. final mass - initial mass = g dissolved solids (this is in the initial volume from part 1!)

6. know and want with know: g -> given volume

want: mg -> 1000cm^3 to get ppm!!

2Q: how to get pH of water sample?

use pH meter

2Q: how to get pH of water sample if pH meter unavailable?

use universal indicator and pH scale

washing soda crystals titration 1. get mass H2O in crystals

1. compare weighed out (given in Q) vs titration results (calculated).

2. get both in same units using know & want

3. subtract grams to get mass H2O in crystals

washing soda crystals titration 2. get % water of crystallisation

aka % H2O crystals = mass H2O crystals (g solved in first part) / mass crystals (given in q) x 100

washing soda crystals titration 3. get mols of water in crystals

convert mass H2O (g solved in first part) into mols using H2O Mr

washing soda crystals titration 4. get value of X in Na2CO3 + XH2O

XH2O / Na2CO3 + XH2O = % H2O crystals (solved in part 2 formula)

2. cross multiply to solve for X

3. round X to nearest whole number

4. sub X value into Na2CO3 + XH2O!

washing soda crystals titration % water of crystallisation is aka

% H2O crystals so mass H2O crystals / mass crystals x 100!

3Q: volatile liquid meaning and example?

liquid that vapourises easily and has a low boiling point e.g. propanone

3Q: how is the mass of the volatile liquid established?

1. using an electronic balance, weight clean, dry conical flask, rubber band, and piece of aluminium foil

2. add small volume of volatile liquid (propanone) into flask and put into a beaker filled with water

3. heat this water bath on a hot plate at close to WATERS boiling point so all the propanone vapourises

4. remove flask from water, let cool at room temperature so propanone vapour can condense and dry sides of conical flask

5. reweigh the flask, rubber band and piece of aluminium foil

6. final mass - initial mass = mass volatile liquid!

3Q: (a) how is the volatile liquid vapourised and (b) how is the temperature of the vapour obtained?

(a) volatile liquid vapourised by placing conical flask submerged in beaker filled with water on a hot plate which is at a temperature close to WATERS BOILING POINT, ensuring volatile liquid vapourises

(b) temperature of vapour obtained using a thermometer to measure the temperature of the water as they can be assumed to be equal!

3Q: how is the pressure of the vapour in the conical flask measured?

barometer to measure atmospheric pressure as vapour pressure = atmospheric pressure

3Q: why is the vapour pressure equal to atmospheric pressure?

due to the pin hole made in the aluminium foil meaning the vapour is in contact with the atmosphere

3Q: how is the volume of propanone vapour found?

by filling the conical flask with water, pouring into a graduated cylinder and reading it's volume

3Q: why is this method for measuring Mr unsuitable for non-volatile liquids?

non-volatile liquids' boiling points are too high so boiling water would not vapourise them

3Q: what instrumental technique is a more accurate method to measure the Mr of volatile and non-volatile liquids as well as solid and gaseous substances?

mass spectrometry (VIASD)

3Q: give 3 errors in this experiment which may lead to inaccurate results?

1. ENTIRE conical flask should be covered in boiling water but this might be difficult to achieve

2. outside of flask and foil mightn't be completely dry when reweighed, skewing results

3. electronic balance may only read to 1-2 decimal places so not v accurate

N.B!!! 3Q: units of PV = nRT

P: pressure in Pa

V: volume in m^3

n: mols

R: gas constant (given front of exam paper!)

T: temperature in kelvins

N.B!!! 3Q: how to convert between pascals (Pa) and kilopascals (kPa)?

Pa --[divide by 1000]--> kPa, then opposite for inverse

N.B!!! 3Q: how to convert between mL, L and m^3?

mL (cm^3) --[divide by 1000]--> litres (l) --[divide by 1000]--> m^3, then opposite for inverse

N.B!!! 3Q: how to convert between temperature and kelvins (K)?

degrees Celsius --[+273]--> kelvins (K), then opposite for inverse

3Q: how to calculate Mr given mols and grams?

think in the exam!

mols x Mr = g, u have mols and grams so just divide both sides by mols to get Mr!