BEGIN SCENARIO A scientist needed to find the equivalent mass and pK, of an unkn
ID: 1036780 • Letter: B
Question
BEGIN SCENARIO A scientist needed to find the equivalent mass and pK, of an unknown acid which is a solid at room temperature. To do this, they titrated samples against a standardized 0.0845 M NaOH solution. Follow the steps below to determine the equivalent mass and pK, of an unknown acid. 1. EQUIVALENT MASS DEFINITION id? Mass of acid in grams needed to supply one mole of H' acid in What is the equivalent mass of an 2. PRELIMINARY TITRATION As the identity, concentration, and molar mass of the acid are not known, the first stage is to estimate an amount of unknown acid solid to weigh out and perform a preliminary titration, and then adjust the amount to weigh out for subsequent titrations. 0.1164 g of the unknown acid was weighed out, and just enough water was added to dissolve all of the TheExplanation / Answer
Unit
Trial 1
Trail 2
Number of equivalents of NaOH (unrounded value) = (volume of NaOH at the equivalence point in L)*(normality of NaOH)
equiv
(25.20 mL)*(1 L/1000 mL)*(0.0845 N)* = 0.0021294
(24.86 mL)*(1 L/1000 mL)*(0.0845 N) = 0.00210067
Rounded Value
equiv
0.0021
0.0021
Number of equivalents of acid (unrounded value)**
equiv
0.0021294
0.00210067
Rounded value
equiv
0.0021
0.0021
Equivalent mass of unknown acid (unrounded value)***
(0.2528)/(0.0021294) = 118.7189
(0.2562)/(0.00210067) = 121.9611
Rounded value
118.7
122.0
*For NaOH, equivalent weight = molecular weight (since one mole of NaOH can furnish only one equivalent of OH-); therefore, normality of NaOH = molarity of NaOH.
** The acid supplies one equivalent of H+ which combines with one equivalent of OH- (from one equivalent of NaOH) to give one equivalent of water as below.
H+ (aq) + OH- (aq)
As per the stoichiometric equation,
1 equivalent H+ = 1 equivalent OH-.
Therefore, 1 equivalent H+ = 1 equivalent acid = 1 equivalent NaOH.
Consequently, equivalents of NaOH at the equivalence point = equivalents of acid.
*** Gram Equivalent mass of unknown acid = (grams of acid taken)/(equivalents of acid)
The equivalent mass is simply the gram equivalent mass expressed without the unit.
Unit
Trial 1
Trail 2
Number of equivalents of NaOH (unrounded value) = (volume of NaOH at the equivalence point in L)*(normality of NaOH)
equiv
(25.20 mL)*(1 L/1000 mL)*(0.0845 N)* = 0.0021294
(24.86 mL)*(1 L/1000 mL)*(0.0845 N) = 0.00210067
Rounded Value
equiv
0.0021
0.0021
Number of equivalents of acid (unrounded value)**
equiv
0.0021294
0.00210067
Rounded value
equiv
0.0021
0.0021
Equivalent mass of unknown acid (unrounded value)***
(0.2528)/(0.0021294) = 118.7189
(0.2562)/(0.00210067) = 121.9611
Rounded value
118.7
122.0
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