A certain hydrate has the formula MgSO_4.xH_2 O. A quantity of the compound (54.

Question

A certain hydrate has the formula MgSO_4.xH_2 O. A quantity of the compound (54.2 g) is heated to drive off the water. If the steam generated exerts a pressure of 24.8 atm in a 2.00 L container at 120.0 degree C, how many moles of water does the hydrate contain? A 6.11 g sample of a Cu Zn alloy reacts with hydrochloric acid to produce hydrogen gas. If the hydrogen gas has a volume of 1.26 Lat 22 degree C and 728 mmHg, what is the percent of Zn in the alloy? How much NaOH (grams) is needed to prepare 546 mL of solution with a pH of 10.00 at 25 degree C?

Explanation / Answer

Question 9.

MgSO4*xH2O

m = 54.2 g of compound

steam -->

PV = nRT

n = PV/(RT) = (24.8)*2/(0.082*(120+273))

n = 1.5391298 moles of H2O

mass of water = mol*MW = 1.5391298*18 = 27.70 g

therefore

MgSO4 mass = 54.2-27.70 = 26.5 g of MgSO4 alone

mol = mass/MW = 26.5/120.366 = 0.22016 mol of MgSO4

ratio

1.5391298 /0.22016 = 6.99 --> 7

so x = 7

MgSO4*7H2O

n = 7 moles of water per mof of hydrate

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