Chem(Help)

用戶73615

2008-05-04 3:50 am

3.72g of hydrated sodium carbonate crystals Na2CO3.xH2O are dissolved in water. The solution is then titrated against 2.00M HCL. Methyl orange is used as and indicator. The results is
Burette reading(cm3) :1st Final reading 31.0, Initial reading 1.0
2nd Final reading 30.0,Initial reading 0.0
3rd Final reading 31.9, Initial reading2.0

a) What is the colour change of the indicator at the end point?
b)Calculate the number of water molecules which combine with one formula unit of Na2CO3.
(Relative atomic masses:H=1,C=12,O=16,Na=23)

Is there any equations, please mention it.

回答 (1)

Uncle Michael

2008-05-04 5:04 am

✔ 最佳答案

(a)
Colour change at the end point is from yellow to orange.

(b)
Vol. of HCl used in run 1 = 31 - 1 = 30 cm3
Vol. of HCl used in run 2 = 30 - 0 = 30 cm3
Vol. of HCl used in run 3 = 31.9 - 2 = 29.9 cm3
Average vol. of HCl used, V = (30 + 30 + 29.9) / 3 = 29.97 cm3 = 0.02997 dm3

Consider the titration:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
No. of moles of HCl used = MV = 2 x 0.02997 = 0.05994 mol
No. of moles of Na2CO3 used = 0.05994 x (1/2) = 0.02997 mol

Na2CO3•nH2O dissolves in water to give Na2CO3 solution.
No. of moles of Na2CO3•nH2O = 0.02997 mol

Molar mass of Na2CO3•nH2O = mass/mol = 3.72/0.02997 = 124 g mol-1
Molar mass of Na2CO3 = 23x2 + 12 + 16x3 = 106 g mol-1
Molar mass of H2O = 1x2 + 16 = 18 g mol-1
Hence, 106 + 18n = 124
n = 1

Therefore, 1 water molecule combines with one formula unit of Na2CO3.

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