Question

A student is given a sample of a manganese(II) chloride hydrate. She weighs the sample in a dry, covered crucible and obtains a mass of 24.747 g for the crucible, cover, and sample. Earlier she had found that the crucible and cover weighed 23.599 g. She then heats the crucible to drive off the water of hydration, keeping the crucible at red heat for about 10 minutes with the cover slightly ajar. She then lets the crucible cool, and finds it has a lower mass; the crucible, cover, and contents then weigh 24.329 g. In the process, the sample was converted to anhydrous MnCl2. a. What was the mass of the hydrate sample? O g hydrate b. What is the mass of the anhydrous MnCl2? C g MnCl2 c. How much water was driven off? Og H20 d. What is the percent by mass of water in the hydrate? Mass of water in sample % water = x 100 Mass of hydrate sample omas H20 C e. How many grams of water would there be in 100.0 g hydrate? H20 How many moles? moles H20 f. How many grams of MnCl2 are there in 100.0 g hydrate? g MnCl, How many moles? (The molar mass of MnCl2 is 125.94 g/mol.) moles MnCl2 D g. How many moles of water are present per mole of MnCl2? h. What is the formula of the hydrate? (Enter the dot symbol using an asterix, *)

Answer 1

Mass of crucible and cover = 23.599 g

Mass of crucible , cover and hydrate = 24.747 g

a) Mass of hydrate = 24.747 g - 23.599 g = 1.148 g

Mass of crucible , cover and salt = 24.329 g

b) Mass of salt = 24.329 g - 23.599 g = 0.730 g

C) Mass of water driven off = Mass of hydrate - Mass of salt = 1.148 g - 0.730 g = 0.418 g

d) % water = ( Mass of water / Mass of hydrate) 100 = (0.418 / 1.148 ) 100 = 36.41

e)

100 g of Hydrate contain 36.41 g water.

We have , No. of moles = Mass / Molar mass

Therefore, No. of moles of water = 36.41 g / 18.01 g/mol = 2.022 mol

f)

grams of MnCl ₂ in hydrate = 100 - 36.41 = 63.59 g

Moles of MnCl ₂ in hydrate = 63.59 g / 125.94 g/mol = 0.5049 mol

g) Ratio of moles of MnCl ₂ : moles of water = 0.5049 : 2.022 = 1: 4.00

4 moles of water are present per mole of  MnCl ₂.

h) Formula of Hydrate : MnCl _{2 *} 4 H₂O