Mole Concept and Limiting Reagent in the Laboratory Classically, chemistry students have found the mole concept and limiting reagents difficult to understand. The following laboratory exercise was developed to direct the student in a step-wise approach to the problem. Prior knowledge and exercises in balancing equations and gram-molecular weights was necessary. The exercise was found to be beneficial and effectivewith general chemistry students at the community college. I t was also best used effectively with above average high school students. A Qutmtitdivt Preripltdion of Barium Phorph*tes Purpose
To use and reinforce the concept of molar relationships and limiting reagent, and also to reinforce lab techniques in analytical weighing and filtering. Rwdion
NarPOl
+ BaCL
+ Bas(P0.)1
+ NaCI
Procedure
The student is to balance equation and note the molar ratios. Each student is assigned a letter from the table below, which tells him how much of each reactant to use. He then fills in the blanks below which helps him to determine the theoretical yield of Bal(P01)~that. can be recovered. After finishing through number 5, the instructor is to check and evaluate the responses. If correct, he is to proceed in the following way. According to the assigned letter weigh out the reagents on a watch glass, remembering to label each. Dissolve each of the reactants in 100 ml water (separate beskerj). With sstirring rod, gently pour and mix the N ~ J P O I solubion illto the BaCb solution. Stir for about. a minute. Allow the precipitate to settle, then decant off the supernatant solut,ion. Filter the remaining solution thoroughly with a gravity funnel or a Buchnor funnel, dry the precipitate, and determine the actual yield. Follow the instructor's direction for reporting results.
+
-
+
Baa(P04)~ NsCl 1. Bdance the reaction: NrtsP04 BaCh 2. There are g /n 1 mole NasPO, There are g !n 1 mole BaCln There are g m 1 mole Ba8(P04). g in 1 mole NaCl There are 3. Assigned Letter 4.
5.
6. 7. 8.
. Thi? iu Ass~gnednumber moles of NaaPO, Assigned number moles of BaCL . This 1s The limiting factor is The number of moles of Bas(P0,)~expected is This k g of Bsw(P0A. The actual number of gms of Bar(PO4)r recovered is Percentage yield Percentage of error
g of NswPO, g of BaCh
Mole Assignment
Letter
Moles N a p 0 1
Moles BaCL
Letter
Moles Na$PO,
Volume 48, Number
Moles BeCL
3, March 1971
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155
