Molar Ratios

Findine the Ratio of Moles of Reactants

lntroducfionA balanced chemical equation gives the mole ratios of ieactants and products for a chemical reaction. If the

fomulas of all reactanh and producti are knowrL it is relatively easy to balance an equation to fnd out uhatthese mole ratios are. When the formulas ofthe products are not kno$r! experimental measwements must bemade to determine lhe ratios.

Concepts. Stoichiometry. Mole ratio. Oxidation-reduction reaction

BockgroundThis experiment uses the method of continuous variations to detemine the mole ratio of tlro reactants

in a chemical reacLion. Several steps are involved. Firsl solutions of the reactants are prepared in which theconcentrations are known. Second, the solutjons are mixed a numbe i of times using different volume ratios ofreactants. Third some propert) of the rcaction that depends on the amolrnt of product formed or on the amountof reactant that renains is measured, This property may be the color intensity due to a reactant or product, themass of a prccipitate that forms, or the volume of a gas elolved.

In the method of continuous vaiations, the total number of moles of reactantJ is kept constant for the seriesof measurements. Each measurement is made with a different mole ratio of reactants. The optimum ratio, whichis the stoichiometric ratio for the .eactants in the balanced chemical equation, should consume the gieatestamount of reactants, form the grcatest amount of product, or generate the most heat and prcduce the maximumtemperature change.

Sodium hypochlorite is an oxidizing agent and all the reactions tested in this experiment are oxidation-reduction reactions. The hypochlorite ion will be reduced as anotier substance is oxidized.

Experimeni OverviewThis experiment uses the method of continuous aariations to deter'lj.ine the mole latio of two reactants,. The

change of temperature is the property to be measurcd. The reactions are all exothermig so the heat producedwill be directly proportional to tlle amount of reaction that occurs. Since the experiment is designed so that thetotal volume of solutions is constnnt for all measurements, the temperature change will also be proportional totle total exfent that the reaction occurs.

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Exp 4: Molar Ratios -2 - Exp 6: FindiDg the Ratio of Moles of Reactants

Pre-Lob Question s (llse a separate sheet ofpaper to ansuer the fottouing questions.)Review the prccedure before answedng questions.

1. The following values were obtained in a continuous va ations experiment designed to find the mole ratio forthe reaction betweel 0.5 M solutions ofAgNO3 and I!CIO{. One of the products is a precipitate.

Experimetrt nl, ACNO" InL IqCroI Grams PrccipitateI 45.0 1.72 15.0 35.03 25.0 25.0 8.34 30.0 20.0 10.05 35.0 15.0 9.96 40.0 10.0 6.67 45.0 5.0 3.3

PIot the data on ggaph paper as outlined in the procedure. Label axes and space the data so that tie gnphreflects the precision of the values given. Use a ruler to dlaw the best-fitting strailht lines through t}le datapointJ and determine the coefficients for the reactants in tie balanced chemical equation,

-AcNo3 + -4crq + Products2. Is there enough data to mal@ a valid conclusion? Why or uhy not?

MoteriolsChemicalsSodium hlpochlorite, NaClO, 0.50 M, 175 mL "Solution B", 0.5 M, 175 mL

EquiptnentSbTofoamo cup Graduated cylinders, 10-mL, 2Thermometer Graduated cylinder,s0-rnlBeakers, 400-mL, 2 Marker

Sofety PrecouiionsBleach is a 5% solution ofsodiwn hwochlorite. This solution is a corosiue liquid; it causes skin bun1s. The

solution reacts uith acid to euolue chlorine gas; uhen heated it e\olues chloine gas, The solution t lfioderatelgtoxic bg ingestion and inhalation. Keep (rua! from skin and clothing. Wotk in a fume hood or oell-uentilatedIab. Wear chenical splash goggles, chemical-resistant glouet, (md a chemical-resistant apron Wash hrndsthoroughly aith soap and uater before learing the laboratory.

Exp 4: Molar Ratios -3 -

Exp 6: rinding the Ratio of Moles ol Reactants

Procedure1. Obtain approximately 175 mL of the NaCIO solution i4 a clean 400-mL beaker and 175 mL of "Solution B"

to another clean 400-mL beaker. label the bealcen.2. Measurc the temperature of the NaCIO solution and of "Solution B." Record the data in the Data'lable. Use

the same tllemometer or a pair of calibnted thermonieters. The solutions should be the same tempeiatule.If they are not, you $rill need to make a correction for tlle tempelatwe difference.

3. Using a cleaD 10-mL gaduated cylinder, measure 5.0 mL of NaCIO solution and pour the solution into astyrofoain cup. Using a clean 50-rnl €raduated cylindet measur€ 45.0 mL of'Solution B" and add this to theStyrofoam cup.

4. Stir with a thermometer, and record the rnaximum tempelahirc reached of t}le 6nal solution in the Data'llble.5. Pour the solution out, rirue the cup and tiermometer, and repeat steps 1-4 using a different ratio of the two

substances, al ays keeping the tota.l volume at 50.0 mL.6. Continue testing various ratios until you have at least thrce fieasurements on each side of the ofie that

g@e the greatest tenperature ditrerence [AT("C)J.7. Plot your data on a graph a5 shown in Fi€ure 1.8. DraN two straight lines that best ft your dat4 and determine where they intersecl Be sure to include the

points at the 0:50 rnl and 50:0 mI ntios. If any points do not fall close to the lines, you should repeat thesemeasurements. Find the stoichiometric mole ratio of reactants frdm the point of intersection on your graph.

nrl NaCIOmL"B"

0 1040

2030

3020

Change in Temlerdture ve6us Reacialt Volum€ Ratios

Figure 1. Graph of Experimental DataDisposol ond Cleonup

Your teacher will provide disposal and cleanup instructions.

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4010

Exp 4: Molar Ratios -4 -

Elp 6: Finding the Ratio of Moles of Re'ctants

Post-Lob Colculqlions ond Anolysis- (llse a separate sheet ofpaper to allsuer the folloaing questians.)1. Explain how the method of contittuons variations is used to determine the mole ratio of reactants ilr a

chemical reaction.2. Why was the total volume of solutions used kept constant in all trials?3. Is it necessary that the concentrations of the two solutions be the same?4. What is meant by the term limiting reagent?5. Which measurement tempenture or volume, limits the precision ofthe data obtained in the experiment?

Explain.6. Which rcactant is the limiting reagent along the upward sloping line of the graph? Which is the limiting

rcagent along the downward sloping line?7. What other physical properties, otier than temperature change, could be used in the method of continuous

variations?8. why is it more accurate to use the point of intersection of the two lines to find the mole lado rather than

the ratio associated 'trith the greatest temperature cha,nge?9. If the two solutions used are not at tlle same initial temperature, a correction must be rnade to find the

correct change in temperafure. How should this be done?