3.1 Gravimetric Analysis Flashcards
gravimetric analysis
used to determine the mass of an element or compound in a substanc
used to determine the mass of an element or compound in a substanc
gravimetric analysis
what must you do to the starting substance before graimetric analysis can take place
the substance is converted into another substance of known chemical composition, which can be readily isolated and purified
how can you convert substances for gravimetric analysis
by precipitation or volatilisation/decomposition
explain how you can convert substances for gravimetric analysis using..
1. precipitation
2. volatilisation/decomposition
- In precipitation conversion the substance undergoes a precipitation reaction. The precipitate is separated from the filtrate.
- In volatilisation conversion the substance is heated and any volatile products (often water) are evaporated.
weighing by difference
the mass of the crucible is measured before adding the substance and the final mass of the substance is determined by subtracting the mass of the crucible from the mass of the crucible and dried substance
why should a blue flame be used when heating to a constant mass
to avoid a build up of soot on the outside of the crucible which could affect the mass
what volume of 0.4 mol/l lithium sulphate is required to make one litre of a solution with a lithium ion concentration of 0.2 mol/l?
= Li2SO4 → 2Li+, 1:2 ratio
= mole = c x v
= 0.2 x 1
= 0.2 moles of Li+ ions
= so we need 0.1 moles of LiSO4
= v = moles / concentration
= 0.1 / 0.4
250ml of lithium sulphate
what volume of 0.5 mol/l magnesium nitrate is required to make 250ml of a solution with a nitrate ion concentration of 0.25 mol/l?
= Mg(NO3)2 → 2 NO3^-, 1:2 ratio
= mole = c x v
= 0.25 x 0.25
= 0.0625 moles
= 0.03125 moles Mg(NO3)2 is required
= v = n / c
= 0.03125 / 0.5
=62.5ml of magnesium nitrate
- a key of mass 12.5g contains copper, silver, and nickel. the key was dissolved in nitric acid and made up to 500ml in a volumetric flask with deionised water.0.5 mol l hydrochloric acid was added to 100ml of the solution to precipitate silver (I) chloride. the precipitate was filtered, washed, and dried. the mass of the precipitate was 0.92g.calculate the percentage by mass of silver in the coin
= mass of AgCl = 0.92g
= moles = mass / gfm
= moles = 0.0064
= moles of Ag in 100ml = 0.0064
= moles of Ag in 500ml = 0.032 moles
= mass of silver = moles x gfm
= 0.032 x 107.9
= 3.46g silver in key
3.46 / 12.5 = 27.7%
- a key of mass 12.5g contains copper, silver, and nickel. the key was dissolved in nitric acid and made up to 500ml in a volumetric flask with deionised water.the filtrate, 100ml, left after precipitating silver (I) chloride is treated with dimethylglyoxime to produce a red precipitate according to the equation below:
7.51g of precipitate is formed, calculate the percentage by mass of nickel in the key.
n = m / gfm
n = 0.026 moles
= n in 100ml = 0.026
= n in 500ml = 0.13
mass of nicket = n x gfm
= 0.13 x 58.7
= 7.63g
7.63 / 12.5 x 100
= 61.1%
= % yield = actual / theoretical x 100
= 65% = 4.76 / theoretical x 100
= theoretical = 4.76 / 0.65
= 7.32g
moles asprin = 7.32 / 180
= 0.04 moles
moles salicylic acid = 0.04
= mass of salicylic acid = 0.04 x 138
= 5.61g is needed
= % yeild = actual / theoretical x 100
= 87 = 5.8 / theoretical x 100
= theoretical = 580 / 87
= theoretical is 6.67g
n = 6.67 / 151
= 0.0442 moles
n of aminophenol = 0.0442
= mass = 0.0442 x 109
= 4.82g required
av. titre = 11.65 / 1000
moles oxalic acid = c x v
= 0.112 x (11.65 / 1000)
= n = 0.0013
moles NaOH
= 0.0013 x 2
= 0.0026 moles
concentration NaOH
= n / v
= 0.0026 / 0.02
= 0.13 moles
= av. titre = 34.85 / 1000
moles of permanganate
= v x c
= (34.85 / 1000) x 0.021
= 0.00073 moles
1:5 ratio
moles of iron = 5 x 0.00073
= 0.0037 moles in 20ml
= 0.018 moles in 100ml
mass of iron
= n x gfm
= 0.018 x 55.8g
= 1.02g iron in tablet
percentage solution
- percentage solution: mass of solute made up to 100cm3 solution
- % solution = (mass of solute / volume of solvent) x 100
- what is the concentration in mass percent of the following:a. 20g of KCl in 600ml of waterb. 32g of NaNO3 in 2000ml of water
a. 3.3%
b. 1.6%
percentage by volume
- percentage by volume: the number of cm3 of solute made up to 100cm3 of solution
- % by volume = (volume of solute / volume of solution) x 100
what is the percentage by volume of methanol if 76cm of methanol is added to 400cm of water
15.8%
what is the volume required to make a 20% solution using 25cm hydrogen peroxide
100ml of water needed
ppm
1mg/kg or 1mg/l
a sample of 300g of water is found containing 38mg of lead. what is the concentration in ppm?
127 ppm
a solution of lead sulphate contains 0.425g of lead sulphate in 100g of water. what is the concentration in ppm?
4250 ppm
- BaCl2 gfm = 208.3
n = m / gfm
2.74 / 208.3
= 0.01315 moles
- 3.2 - 2.74 = 0.46g
- n = m / gfm
0.46 / 18 = 0.0256
- 0.01315: 0.0256
1:2 ratio
