化學問題(英) 請大家幫幫忙 >0

化學問題(英)

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1. What is the molar concentration of a solution in which 75.02g of Na2CrO4 was completely dissolved in 100.00 mL of water and brought to a final solution volume of 250.00 mL?

2.What is the molar concentration of a H3PO3 solution if 150.00 mL of the solution was required to neutralize 75.00 mL of NaOH whose contration was 1.50 molar?

3. What is the molar concentration of a 70.00% by weight isopropyl acohol, C3H8O, solution that has a density of 0.9681 g/mL?

4. A 0.4970 mole sample of a metal oxide weights 50.67g. What is the identity of the metal,M, if the general formula of the metal oxide is M2O3?

a) Fe b) Cr c) Ca d) Al

5. What is the percent composition of potassium in K2CrO4?

1 個解答

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  • 1 十年前
    最愛解答

    1. value of 100ml is useless. don't be distracted -- only the final volume is useful!

    no. of mole of solute = mass / molar mass = 75.02 / (23x2 + 52 + 16x4)

    = 0.4631 mole

    molar concentration = no. of mole / volume = 0.4631 / (250/1000)

    = 1.85 M.

    2. write the equation:

    H3PO3 + 2NaOH ------> Na2HPO3 + 2H2O

    [note that H3PO3, phosphorous acid / phosphorus(III) acid is dibasic acid !!]

    no. of mole of NaOH consumed = molarity x volume = 1.50 x (75/1000)

    = 0.1125 mole

    mole ratio of NaOH : H3PO3 = 2:1

    no. of mole of acid = 0.1125 / 2 = 0.05625 mole

    molarity = no. of mole / volume = 0.75 M.

    3. assuming there's 1000ml of solution, then mass of isopropyl alcohol

    = density x volume x %-by-mass = 0.9681 x 1000 x 70%

    = 677.67 g

    no. of mole in 1000ml = mass / molar mass = 677.67 / (12x3 + 1x8 + 16) = 11.2945 mole

    molarity = no. of mole / volume = 11.2945 / (1000/1000) = 11.29 M.

    4. molar mass of metal oxide = mass / no. of mole = 50.67 / 0.4970

    = 101.95 g/mol.

    if molar mass of metal = y, then 2y + 16x3 = 101.95

    y = 26.98 g/mol

    dig out molar mass of each metal mentioned ...... metal is aluminium.

    5. assuming there's 1000g of K2CrO4.

    no. of mole = 1000/(39.1x2 + 52 + 16x4) = 5.1493 mole

    every mole of salt gives 2 mole of potassium.

    thus no. of mole of potassium = 5.1493 x 2 = 10.2987 mole

    mass of potassium = 10.2987 x 39.1 = 402.6777 g

    percentage by mass = mass of potassium in this sample / mass of whole sample

    = 402.6777 / 1000 = 40.2%.

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