? 發問於 科學及數學化學 · 10 年前

有關chem titration既問題

Before titration, the conical flask should be cleaned by washing with water and it does not need to be dried before use.Explain why it does not need to be dried.

2 個解答

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  • 10 年前
    最愛解答

    It dose not change the no.of moles of solution in the conical flask,which

    uses up the same amount of titrant whether it is dried or not.

  • 匿名
    10 年前

    during titration

    you need to calculate the exact amount of substances reacted

    in the conical flask

    substances is transferred by a pipette and a pipette filler

    pipette can transfer 25.00cm^3 of the solution exactly

    with the step that rinse the pipette by distilled water and the soulution before use it

    we can calculate the exact no. of mol of substance containing

    for example

    when we transfer 5M sodium hydroxide solution by a 25.00cm^3 pipette to the conical flask

    we can know that no. of mol of NaOH in the flask is

    5M x (25.00/1000) dm^3 = 0.125mol

    from the above,

    we can exactly know the no. of mol of NaOH in the conical flask

    besides

    when the indicator (in the solution) change its colour (reach the end point)

    that means the 0.125mol of NaOH is just all reacted

    after we have this data :

    reasonable average volume of the titrant (solution in burette)

    no. of mol of substance of the titrand ( solution in conical flask)

    Equation of the reaction

    Then we can know the molarity of the solution in burette

    for example: 30.00 cm^3 of unknown molarity of HCl is needed to let the 25.00 cm^3 5M NaOH for complete neutralization

    then we have

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

    no. of mol of HCl : M of HCl x (30.00/1000) dm^3

    no. of mol of NaOH : 5M x (25.00/1000) dm^3 = 0.125mol

    According to the equation, no. of mol of NaOH : no. of mol of HCl = 1:1

    therefore, the molarity of HCl :

    M of HCl x (30.00/1000) dm^3 = 0.125mol

    M of HCl = 4.167 M

    2011-03-21 06:40:31 補充:

    From th above, we can know that the important point is that the no. of mol of titrand (solution in conical flask) should be exacted!

    and the molarity is not impotant ! (mainly because we will use the calculated no. of mol of substances in conical flask to find the unknown in the titration)

    資料來源: self
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