gravimetric analysis & stoichiometry

Question 1

strategy to answer stoichiometry questions?

Answer 1

1. calculate the moles of the known substance
2. use the mole ratio to work out moles of unknown substance
3. answer the question eg. mass, volume, concentration

Question 2

what is the limiting reagent?

Answer 2

the reactant that is completely consumed in the reaction, the reagent that runs out first in the chemical reactions

USE THIS IN ALL STOICHIOMETRIC CALCULATIONS

Question 3

how to determine limiting/excess reagents?

Answer 3

1. write balanced equation if not given one
2. find moles of substances
3. find the theoretical molar ratio (from equation)
4. find the actual molar ratio
5. work out limiting reagent

Question 4

what is gravimetric analysis?

Answer 4

a quantitative analysis technique through which the amount of the analyte can be determined through the measurement of mass before/after a physical or chemical change

depends on comparing the masses of the two compounds containing analyte

Question 5

what is the analyte?

Answer 5

the ion or solution being analysed

Question 6

solubility rules

Answer 6

all NITRATES (NO3-) are soluble
all group 1 and NH4+ (ammonium) salts are soluble
oxides, carbonates, nitrides, sulphides are INSOLUBLE except with the above

Question 7

what is the process of using grav analysis w/ precipitation?

Answer 7

1. ensure that the volume of the analyte is known
2. add excess reagent to precipitate out all of the ‘active’ ions in the analyte
3. filter out the precipitate and discard the filtrate
4. rinse the residue (precipitate) with distilled water and dry to constant mass
5. weigh the precipitate and perform necessary calculations

Question 8

define precipitate

Answer 8

a solid that is formed in a reaction between two dissolved substances/gases

Question 9

define residue

Answer 9

the solid remaining in filter paper during filtration

Question 10

define filtrate

Answer 10

the liquid that passes through the filter paper during filtration

Question 11

what is dry to constant mass?

Answer 11

a process in which a solid is weighed, dried and re-weighed until the mass no longer changes. this ensures that all of the excess water has been completely removed

Question 12

sources of error and impacts (part 1)

Answer 12

1. solubility or insolubility of precipitate -> can lose some precipitate in filtrate, which is an incomplete precipitation so LOWER EXPERIMENTAL CONTENT that expected (lower measured mass)
2. losses through splashing, inefficient rinsing, inefficient filtering -> incomplete transfer of precipitate so LOWER MEASURED MASS
3. extent of drying the solid -> incomplete drying, so there is still some water in the precipitate so HIGHER MEASURED MASS

Question 13

sources of error and impacts (part 2)

Answer 13

4. has all the active ingredient been precipitated out? -> incomplete precip. = LOWER MEASURED MASS, lower experimental content than expected
5. measurement errors -> random outcomes due to inaccuracies
6. variations of solubility with different temps -> incomplete precipitation
   so LOWER MEASURED MASS