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Thread: past question help

  1. #1
    Cadet
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    past question help

    Citric acid, the predominant acid in lemon juice, is a triprotic acid. A student titrated 25.0 mL samples of lemon juice with 0.550 mol L–1 NaOH. The mean titration volume was 29.50 mL. The molar mass of citric acid is 192.12 g mol–1.

    What was the concentration of citric acid in the lemon juice?

    (A) 1.04 g L–1
    (B) 41.6 g L–1
    (C) 125 g L–1
    (D) 374 g L–1


    also
    A household cleaning agent contains a weak base with the formula NaX. 1.00 g of this compound was dissolved in water to give 100.0 mL of solution. A 20.0 mL sample of the solution was titrated with 0.100 mol L–1 hydrochloric acid, and required 24.4 mL of the acid for neutralisation.

    What is the molar mass of the weak base?

    (A) 82.0 g
    (B) 84.0 g
    (C) 122 g
    (D) 410 g
    Last edited by zeee1999; 4 Oct 2017 at 12:00 PM.

  2. #2
    Junior Member Sp3ctre's Avatar
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    Re: past question help

    1. n(NaOH) = cv
    n(NaOH) = 0.550 mol/L * 0.0295 L
    n(NaOH) = 0.016225 mol

    n(citric) = 0.016225/3 (because it is triprotic)
    n(citric) = 5.41*10^-3 moles

    c(citric) = 5.41*10^-3 mol / 0.025 L
    c(citric) = 0.2163... mol/L
    c(citric) = 0.2163 * 192.12 g/L
    c(citric) = 41.56196

    Therefore the answer is B. Similar working is required for the second question
    HSC 2017: English Adv - Maths Ext 1 - Maths Ext 2 - Chemistry - Physics
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