Experiment 8 - Limiting Reactants

what crystals contribute to the formation of kidney stones?

calcium oxalate crystals

what are the 2 objectives of the experiment?

1. determine the limiting reactant in a mixture of two soluble salts

2. to determine the percent composition of each substance in a salt mixture

percent composition

the mass ratio of a component of a mixture or compound to the total mass of the sample times 100

(mass of one component/total mass of sample)*100

What 2 factors affect the yield of products in a chemical reaction?

(1) moles of reactants

(2) the percent yield of the reaction

many experimental conditions (temp and pressure) can be adjusted to increase the yield of a desired product in a chemical rx, but bc the chemicals react according to fixed mole ratios (stoichiometrically), only a limited amount of ___________ can form from measures amounts of starting material

product

percent yield formula

actual yield/theoretical yield x 100

stoichiometrically

by a study of a chemical rx using a balanced equation

limiting reactant

the reactant that determines the amount of product generated in a chemical reaction

name of CaCl2*2H2O

calcium chloride dihydrate

name of K2C2O4*H2O

potassium oxalate monohydrate

formula of potassium oxalate monohydrate

K2C2O4*H2O

formula of calcium chloride dihydrate

CaCl2*2H2O

CaCl2 2H2O(aq) + K2C2O4 H2O(aq) ------->

CaC2O4 * H2O(s) + 2 KCl(aq) + 2H2O(l)

calcium oxalate monohydrate, CaC2O4 * H2O, the product, is an ________ compound, but is found naturally in a number of diverse locations

examples of where it is found?

insoluble

plants (rhubarb leaves, agave, and in small amounts in spinach

what is the cause of most kidney stones?

in small doses, it causes a severe reaction where?

calcium oxalate monohydrate, CaC2O4 * H2O

to the lining of the digestive tract

for the reaction system in this experiment, both the calcium chloride and potassium oxalate are __________ salts, but the calcium oxalate is insoluble

soluble

ionic equation

a chemical equation that presents ionic compounds in the form in which they exist in aqueous solution

spectator ions

cations or anions that do not participate in any observable or detectable chemical rx

net ionic equation

Includes only those ions that undergo a chemical change in a reaction in an aqueous solution

what is the net ionic equation of this reaction?

Ca^2+(aq) + C2O4^2-(aq) + H2O(l) -----> CaC2O4*H2O(s)

how is calcium oxalate monohydrate below 90 degrees C?

at temperatures above 110 degrees C?

thermally stable

forms the anhydrous salt, CaC2O4

what anhydrous salt is formed from calcium oxalate monohydrate?

CaC2O4

one mole of Ca2+ (from one mole of CaCl2 •2H2O, molar mass = 147.02 g/mol) reacts with one mole of C2O4 2- (from one mole of K2C2O4 •H2O, molar mass = 184.24 g/mol) to produce

one mole of CaC2O4 •H2O (molar mass = 146.12 g/mol)

In part A of this experiment, the solid reactant salts (CaCl22H2O and K2C2O4H2O) form a ____________ mixture of unknown composition

the mass of the solid mixture is measured and then added to _____. what does this form?

How is the CaC2O4*H2O precipitate collected? what's done after this?

heterogeneous

water ; forms insoluble CaC2O4*H2O

via gravity filtration ; it is dried and its mass is measured

the percent composition of the salt mixture is determined by first doing what?

in part b, the limiting reactant for the formation of solid calcium oxalate monohydrate is determined from what two precipitation tests of the final reaction mixture from Part A?

first testing for the limiting reactant

1. the mixture is tested for an excess of calcium ion with an oxalate reagent—observed formation of a precipitate indicates the presence of an excess of calcium ion (and a limited amount of oxalate ion) in the salt mixture

2. the mixture is also tested for an excess of oxalate ion with a calcium reagent—observed formation of a precipitate indicates the presence of an excess of oxalate ion (and a limited amount of calcium ion) in the salt mixture

Supernatant

the clear solution that exists after the precipitate has settled

Observations from tests on the supernatant solution in Part B determine...

An analysis of the data provides the determination of the ___________ ________________ of the salt mixture

which salt in the mixture is the limiting reactant

percent composition

how many trials are recommended for this exp?

2

to hasten the analyses, measure the mass of duplicate unknown solid salt mixtures in clean .... and simultaneously follow the procedure for each

label the beakers accordingly for trial 1 and 2 to avoid...

obtain about how many grams of an unknown CaCl2 •2H2O/K2C2O4 •H2O salt mixture?

150- or 250-mL beakers

the intermixing of samples and solutions

2-3 grams

(proced. overview: prep. salt mixture A1a)

measure the mass (+/-0.001g) of beaker 1 and record for trial 1. Transfer ___g of the salt mixture to the beaker, measure, and record the ___________ mass. repeat for trial 2, beaker 2

1 ; combined

(proced. overview: prep. salt mixture A1b)

Fill a 400-mL beaker w ____________ water. Test w pH paper.

If the water is acidic, adjust it to be just basic with drops of 6M ____. If already basic to pH paper, then no addition is necessary.

deionized

NH3

(proced. overview: prep. salt mixture A1c)

add _____mL of the deionized water from 1B to the salt mixture in beaker 1

stir the mixture with a stirring rod for how many minutes? then allow the precipitate to settle. leave the stirring rod in the beaker. repeat for trial 2

150 mL

2-3 mins

(proced. overview: digest the precip. A2a)

heat. Cover the beaker with a _______ ______ and warm the solution on a hot plate to a temperature not to exceed ___°C for _____ minutes.

Periodically stir the solution and, in the meantime, proceed to Part A.3

watch glass

75 degrees C

15 mins

(proced. overview: digest the precip. A2b)

cool. After ~15 minutes, remove the heat and allow the precipitate to settle; the solution does not need to cool to ______ ______________

room temperature

why doesn't calcium oxalate precipitate in an acidic solution?

because of the formation of H2C2O4- (an ion that does not precipitate with Ca^2+

Digesting a precipitate in warm water promotes .....

The result is a more efficient filtering process.

dissolving of the smaller precipitate particles followed by their re-precipitating onto the existing larger particles

(proced. overview: digest the precip. A2c)

Wash water. while the precipitate is settling, heat (70-80°C) _____mL of deionized water for use as wash water in Part A.5.

30 mL

(proced. overview: Set up gravity filtering apparatus A3)

Place your initials (in pencil) and "Trial 1" on a piece of ....fold, and tear off its corner.

Measure and record its mass (±0.001 g). Seal the filter paper into the filter funnel with a small amount of ___________ _______.

Discard the deionized water from the receiving flask.

Whatman No. 42 or Fisherbrand Q2 filter paper

deionized water

(proced. overview: withdraw and save supernatant A4)

Once the precipitate has settled and the supernatant has cleared in Part A.2b, use a dropping pipet to withdraw enough supernatant to half-fill two ___-mm test tubes labeled "1" and "2." Save for Part B.

75 mm

(proced. overview: Filter the CaC2O4 •H2O precipitate A5)

While the remaining solution of the salt mixture from Part A.4 is still warm, quantitatively transfer the _________ to the filter (Figure 8.2).

Transfer any precipitate on the wall of the beaker to the filter with the aid of a rubber policeman; wash any remaining precipitate onto the filter with three or four __-mL volumes of warm water (from Part A.2c

precipitate

5 ml

(proced. overview: Dry and measure the amount of CaC2O4 •H2O precipitate A6)

Remove the filter paper and precipitate from the _______ funnel. Air-dry the precipitate on the filter paper until the next laboratory period or dry in a <110°C constant temperature drying oven for at least ___ hour or overnight.

Determine the _________ mass (±0.001 g) of the precipitate and filter paper. Record. Repeat for Trial 2

filter ; 1 hour

combined

(proced. overview: formula of precip. A7)

If the precipitate is air-dried, the precipitate is ___________________

if oven-dried at ≥110°C, the precipitate is the ___________ _____________

CaC2O4 •H2O

anhydrous CaC2O4

rubber policeman

a spatula-like rubber tip attached to a stirring rod

Whatman No. 42 and Fisherbrand Q2 filter papers are both fine-porosity filter papers; a fineporosity filter paper is used to .....

reduce the amount of precipitate passing through the filte

(proced. overview: clarify the supernatant B1)

___________ the 2 collected supernatant samples from PartA4

centrifuge

(proced. overview: Test for excess C2O4^2- B2)

Add ____ drops of (test reagent) 0.5 M CaCl2 to the supernatant liquid in test tube 1.

If a precipitate forms, the ________ is in excess and Ca2+ is the limiting reactant in the original salt mixture

two

C2O4^2-

(proced. overview: Test for excess Ca^2+ B3)

Add two drops of (test reagent) ____ M K2C2O4 to the supernatant liquid in test tube 2.

If a precipitate forms, the ______ is in excess and C2O4^2- is the limiting reactant in the original salt mixture

0.5 M

Ca2+

where should you dispose the calcium oxalate, including the filter paper?

where should you dispose of the waste solutions?

waste solids container

in the waste liquids container

cleanup: rinse each beaker w small portions of _____ water and discard in the waste liquids container

rinse twice w tap water and twice w ______ water and discard in the sink

warm

deionized

all reactions other than __________ reactions have limiting reactants

decomposition