Lesson ten - Diễn đàn
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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson ten (Titration)

Lesson ten
ProfVietanhNgày: Thứ ba, 2010-04-27, 5:19 PM | Tin nhắn # 1
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Titration

There are two ways to make a solution of known molarity. The first way is to make the solution carefully, using an accurately weighed amount of solute dissolved in solvent to an accurately calibrated volume. Often though, it’s more convenient to make up a solution quickly, using an estimated amount of solute and an estimated amount of solvent, and then determine the solution’s exact molarity by titration.

Titration is a procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a standard solution of another substance, whose concentration is known. By finding the volume of the standard solution that reacts with the measured volume of the first solution, the concentration of the first solution can be calculated. (It’s necessary, though, that the reaction go to completion and have a yield of 100%.)

To see how titration works, let’s imagine that we have an HCl solution (an acid) whose concentration we want to find by allowing it to react with a base such as NaOH in what is called a neutralization reaction. (We’ll learn more about acid–base neutralization reactions in the next chapter.) The balanced equation is:

NaOH (aq) + HCl (aq) -> NaCl (aq) + H2O (l)

We begin the titration by measuring out a known volume of the HCl solution and adding a small amount of an indicator such as phenolphthalein, a compound that is colorless in acidic solution but turns pink in basic solution. Next, we fill a calibrated glass tube called a buret with an NaOH standard solution of known concentration, and we slowly add the NaOH to the HCl until the phenolphthalein just begins to turn pink, indicating that all the HCl has reacted and that the solution is starting to become basic. By then reading from the buret to find the volume of the NaOH standard solution that has reacted with the known volume of HCl, we can calculate the concentration of the HCl.

Let’s assume, for example, that we take 20.0 mL of the HCl solution and find that we have to add 48.6 mL of 0.100 M NaOH from a buret to obtain complete reaction. Using the molarity of the NaOH standard solution as a conversion factor, we can calculate the number of moles of NaOH undergoing reaction:
Moles of NaOH = 0.0486 L NaOH x 0.0100 mol NaOH/1 L NaOH = 0.00486 mol NaOH

According to the balanced equation, the number of moles of HCl is the same as that of NaOH:

Moles of HCl = 0.00486 mol NaOH x 1 mol HCl/1 mol NaOH = 0.00486 mol HCl

Dividing the number of moles of HCl by the volume then gives the molarity of the HCl:
HCl molarity = 0.00486 mol HCl/0.0200 L HCl = 0.243 M HCl

Vocabulary
accurately: chính xác, đúng đắn
calibrate: định cỡ, chia độ
convenient: thuận lợi, thuận tiện
let’s assume: giả sử
obtain: đạt được, thu được, giành được
conversion factor: hệ số chuyển đổi


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson ten (Titration)
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