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 .

in the given question, the partial pressure is calculated by using the ideal gas equation, that is, pv = nrt

here, p is pressure, v is volume = 25 l, n is number of moles = 0.0104 moles, r is gas constant = -0.0821 l.atm / mol.k

t is temperature = 273 k

p = nrt / v

p = (0.0104 mol × 0.0821 l.atm / mol.k × 273 k) / 25 l

= 0.0093 atm