[15分] CE chem mc urgent !

👨🏻‍🏫 學術台化學

恩樂

2009-04-13 11:08 pm

The formula of a solid tribasic acid is H3X . 47.1g of the acid is dissolved in some distilled water and the solution is then disluted to 1000cm^3 with disstilled water
25cm^3 of the diluted solution requires 20cm^3 of 1.80M sodium hydroxide solution for complete neutraluzation. What is the relative molecular mass of H3X ?
A 10.9
B. 32.7
C. 52.3
D. 98.1

Illustration !

更新1:

both are good

回答 (2)

老爺子

2009-04-13 11:33 pm

✔ 最佳答案

The answer is D.

Consider the titration of 25 cm3 of the H3X solution:
H3X + 3NaOH → Na3X + 3H2O
No. of moles of NaOH = 1.8 (20/1000) = 0.036 mol
No. of moles of H3X = 0.036 (1/3) = 0.012 mol
Concentration of H3X solution = 0.012 / (25/1000) = 0.48 M

Consider the preparation of 1000 cm3 solution using 47.1 g of H3X:
No. of moles of H3X used = 0.48 (1000/1000) = 0.48 mol
Mass of H3X used = 47.1 g
Molar mass of H3X = 47.1/48 = 98.1 g/mol
Relative molecular mass of H3X = 98.1
=

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[匿名]

2009-04-13 11:37 pm

Number of moles of NaOH= 1.8 x 20/1000
= 0.036

Number of moles of H3X in 25cm^3 = 0.036/3
=0.012

Number of moles of H3X in 1000cm^3=0.012 x10000/25
= 0.48

Let x be the relative mass of X.
Number of moles of H3X = mass of H3X/molar mass of H3X
0.48 = 47.1 / (3 + x)
x = 95.125

Relative molecular mass of H3X = 3+95.125
= 98.125
= D

參考: chem book

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