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Zorluk: ZorWater of Crystallization, Deliquescence, Efflorescence, and Hygroscopy

A 14.3 g14.3\text{ g} sample of hydrated sodium trioxocarbonate(IV), Na2CO3⋅xH2O\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}, was heated strongly in a crucible to constant mass. After complete heating, the mass of the remaining anhydrous residue was found to be 5.3 g5.3\text{ g}. Given the relative atomic masses (Na=23,C=12,O=16,H=1)(\text{Na} = 23, \text{C} = 12, \text{O} = 16, \text{H} = 1), what is the value of xx in the formula of the hydrated salt?

  1. 1010Cevap
  2. B
    77
  3. C
    55
  4. D
    11

Cevap

The value of xx is 1010, giving the formula Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}.
Heating to constant mass completely removes the water of crystallization (9.0 g9.0\text{ g} of H2O\text{H}_2\text{O}, equal to 0.50 mol0.50\text{ mol}). The remaining 5.3 g5.3\text{ g} of anhydrous Na2CO3\text{Na}_2\text{CO}_3 equals 0.05 mol0.05\text{ mol}. Dividing 0.50 mol0.50\text{ mol} by 0.05 mol0.05\text{ mol} yields a mole ratio of 1010, which means x=10x = 10.

Adım Adım Çözüm

1
Calculate the mass of water lost during heating
Mass of H2O=14.3 g−5.3 g=9.0 g\text{Mass of H}_2\text{O} = 14.3\text{ g} - 5.3\text{ g} = 9.0\text{ g}
The loss in mass upon heating to constant mass corresponds directly to the driven-off water of crystallization.
2
Determine the molar masses of anhydrous Na2CO3\text{Na}_2\text{CO}_3 and H2O\text{H}_2\text{O}
Molar mass of Na2CO3=(2×23)+12+(3×16)=106 g/mol\text{Molar mass of Na}_2\text{CO}_3 = (2 \times 23) + 12 + (3 \times 16) = 106\text{ g/mol}; Molar mass of H2O=(2×1)+16=18 g/mol\text{Molar mass of H}_2\text{O} = (2 \times 1) + 16 = 18\text{ g/mol}
Molar masses are required to convert the given masses into mole quantities.
3
Calculate the number of moles of anhydrous salt and water
Moles of Na2CO3=5.3 g106 g/mol=0.05 mol\text{Moles of Na}_2\text{CO}_3 = \frac{5.3\text{ g}}{106\text{ g/mol}} = 0.05\text{ mol}; Moles of H2O=9.0 g18 g/mol=0.50 mol\text{Moles of H}_2\text{O} = \frac{9.0\text{ g}}{18\text{ g/mol}} = 0.50\text{ mol}
Moles equal mass divided by molar mass.
4
Calculate the mole ratio to find xx
x=Moles of H2OMoles of Na2CO3=0.500.05=10x = \frac{\text{Moles of H}_2\text{O}}{\text{Moles of Na}_2\text{CO}_3} = \frac{0.50}{0.05} = 10
The coefficient xx represents the stoichiometric ratio of moles of water of crystallization per mole of anhydrous salt.

Anahtar Kavram

Determination of Water of Crystallization in Hydrated Salts
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Konular

Water of Crystallization, Deliquescence, Efflorescence, and Hygroscopy

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