What is the percent purity of the calcium hydroxide in this titration problem?

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Discussion Overview

The discussion revolves around calculating the percent purity of calcium hydroxide (Ca(OH)2) in a titration problem involving hydrochloric acid (HCl). Participants explore various methods and equations relevant to titration, including molarity and normality, while addressing the calculations involved in determining purity from a given sample.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant outlines a step-by-step calculation to determine the number of mmol of HCl used in the titration and attempts to convert this to the mass of Ca(OH)2, questioning the resulting percent purity as exceeding 100%.
  • Another participant suggests using the Henderson-Hasselbalch equation, although they admit limited knowledge of titration.
  • Some participants propose using the molarity equation and normality concepts to find equivalents in the reaction, with one asserting that normality is necessary for titration calculations.
  • A participant emphasizes the importance of writing out the titration reaction to determine the stoichiometry between acid and base, suggesting a proportional approach to find the moles of alkali present.
  • Concerns are raised about the calculations involving the volume of the Ca(OH)2 solution and the conversion factors used, with calls for clarification on specific steps.

Areas of Agreement / Disagreement

There is no consensus on the correct approach to solving the problem, with multiple competing views on the appropriate equations and methods to use. Participants express uncertainty about the calculations and the validity of different proposed methods.

Contextual Notes

Participants highlight potential issues with assumptions made in calculations, such as the volume of the Ca(OH)2 solution and the conversion factors used. There are unresolved mathematical steps and differing interpretations of how to apply titration principles.

Who May Find This Useful

This discussion may be useful for students learning about titration techniques, stoichiometry, and the calculations involved in determining the purity of chemical compounds in a laboratory setting.

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Homework Statement



A 0.2600-g sample of impure Ca(OH)2 is dissolved in enough water to make 45.70 mL of solution. 20.00 mL of the resulting solution is then titrated with 0.2455-M HCl. What is the percent purity of the calcium hydroxide if the titration requires 9.88 mL of the acid to reach the endpoint?



Homework Equations





The Attempt at a Solution



1) determine the number of mmol of HCl

20.00 mL*(.2455 mmol/1 mL) = 4.91 mmol

2) Convert mmol HCl to OH^1- using the notion that H^+ + OH^1- => H2O

4.9 mmol H^+ *(1mmol OH^1-/1 mmol H^1+) = 4.9 mmol OH^1-

3) Convert the mmol of OH^1- in 45.70 mL of the solution of Ca(OH)2

4.9 mmol OH^1- * (1mmol Ca(OH)2/1mmol (OH)^1-) *(74.1 mg Ca(OH)2/1mmol Ca(OH)2) = 363.09 mg Ca(OH)2

4.) I used the amount of Ca(OH)2 in 9.88 mL of solution of mass of Ca(OH)2

363.09 mg Ca(OH)2 * ((1000 mL/9.88 mL)) = 36750 mg = 36.75 g Ca(OH)2

5.) Use the mass of Ca(OH)2 and mass of pellets to now determine percent purity

36.75 g Ca(OH)2 /.2600 * 100 = 14134.61538 %

Shouldn't the percent purity be smaller than 100 %
 
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i don't know much of titration but i do know an equation that may help you.
to solve for a Henderson-Hasselbalch equation:
pH=pKa+log(base/acid) (fraction)
maybe you can go study a little more on the subject. it will also help if you show us your own attempts to the questions.
(visit my blog!)
 
Last edited:
kaisxuans said:
i don't know much of titration but i do know an equation that may help you.
to solve for a Henderson-Hasselbalch equation:
pH=pKa+log(base/acid) (fraction)
maybe you can go study a little more on the subject. it will also help if you show us your own attempts to the questions.
(visit my blog!)

those were my own attempts. There was a similar problem like the one in my homework in my textbook , and so I used their explanations of the steps of the problems because I thought it would make it more clear to the chemistry tutors

We have started on the Henderson -Hasselbalch equation yet.
 
Oh i see
im really very sorry but i cannot help you in any other way. if you see my blog or profile you will know what i am...so i can't help you
SORRY
oh and you wrote wrong it is HAVENT
 
Last edited:
kaisxuans said:
Oh i see
im really very sorry but i cannot help you in any other way. if you see my blog or profile you will know what i am...so i can't help you
SORRY

its okay . I appreciate you voluntary to help me .
 
Use molarity equation m1v1=m2v2.First find molarity of Ca(OH)2 .You'll get the vol. of HCL used this way.

Hope i am correct.
 
I am not sure about the above one but if u convert molarity into normality then using the concept that the no.of equivalents(of any compound) in any reaction remains the same you can get the answer.
 
Molarity won't work since this is titration reaction.So here is my solution as it is tough to explain.If u have any problem in understanding please post.

Ca(OH)2 +2HCl -> CaCl2 + 2H20

Valence factor of Ca(OH)2 = 2

Valence factor of HCl = 1

Normality of HCl = 0.2455 = 0.2455N

Equivalents of HCl in 9.88ml = 0.2455 * 9.88 = 2.42554meq

This implies eq in 20ml impure Ca(OH)2 solution = 2.42554meq
Equivalents in 45.7ml = 5.54meq

This implies pure Ca(OH)2 in 0.26g sample has 5.54meq

Mass of 5.54meq = 5.54/1000 * Eq weight of Ca(OH)2 = 0.205g

Percent Purity= Pure/Impure *100

I guess you can work that out :D
 
Start at the beginning... write out the titration reaction.
 
  • #10
Benzoate said:
4.9 mmol OH^1- * (1mmol Ca(OH)2/1mmol (OH)^1-) *(74.1 mg Ca(OH)2/1mmol Ca(OH)2) = 363.09 mg Ca(OH)2

1mmol/1mmol?

363.09 mg Ca(OH)2 * ((1000 mL/9.88 mL)) = 36750 mg = 36.75 g Ca(OH)2

Why 1000 mL? What was your Ca(OH)2 solution volume?


 
  • #11
as chemisttree said you should be writing the equation first. you will then be able to determine how many moles of acid reacts with how many moles of alkali/base.

then calculate the number of moles of acid which reacted. use a simple proportion to find out the number of moles of alkali which reacted.(use the equation for that)

this amount of alkali will be present in 20 mL of solution. but you made 45.7 mL solution. you should be calculating the number of moles of the alkali in this volume, using simple proportion.

now convert this amount in mole to mass in grams.

from the 0.2600 g of solid, only the above calculated mass of Ca(OH)2 is present.
do your % purity and you're home and dry!
 
  • #12
simple proportion is most often easiest way to solve titration problems... formulas tend to be too confusing and you can easily make mistakes...
 

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