It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration). A 43.19 gram sample of a hydrate of MgBr2 was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 27.21 grams of the anhydrous compound remained. What is the formula of the hydrate?

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Explanation:

Let denote the number of formula units for every formula unit. The formula of the hydrate would be .

Look up the relative atomic mass of each element on a modern periodic table:

Formula mass:

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The anhydrous compound would be . There was of this compound. Calculate the number of moles of formula units in that much of this compound:

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During heating, of water escaped from the hydrate. Calculate the number of formula units in that of :

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In other words, approximately of formula units (and hence the same number of moles of formula units) contain approximately of formula units. Calculate the ratio between the two:

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Hence, the formula of the hydrate would be .

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2