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Difficulty: EasyRelative Atomic Mass, Relative Molecular Mass, and Carbon-12 Scale

What is the relative molecular mass of hydrated sodium trioxocarbonate(IV) crystals, Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O? [Na=23,C=12,O=16,H=1][Na = 23, C = 12, O = 16, H = 1]

  1. 286Answer
  2. B
    106
  3. C
    124
  4. D
    268

Answer

The relative molecular mass of hydrated sodium trioxocarbonate(IV), Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O, is 286.
The relative molecular mass of a hydrated compound is calculated by taking the sum of the relative atomic masses of all constituent elements in both the anhydrous salt and the water molecules of crystallization. For Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O, (23×2)+12+(16×3)+10×[(1×2)+16]=46+12+48+180=286(23 \times 2) + 12 + (16 \times 3) + 10 \times [(1 \times 2) + 16] = 46 + 12 + 48 + 180 = 286.

Step-by-Step Solution

1
Calculate the relative formula mass of anhydrous Na2CO3Na_2CO_3
(23×2)+12+(16×3)=46+12+48=106(23 \times 2) + 12 + (16 \times 3) = 46 + 12 + 48 = 106
Sum the relative atomic masses of all atoms present in the anhydrous salt portion.
2
Calculate the relative mass of the water of crystallization (10H2O10H_2O)
10 \times [(1 \times 2) + 16] = 10 \times 18 = 180
Multiply the relative molecular mass of one water molecule (18) by the coefficient 10.
3
Add the mass of the anhydrous salt to the mass of the water of crystallization
106 + 180 = 286
Combine both components to obtain the total relative molecular mass of the hydrated crystal.

Key Concept

Relative Molecular Mass of Hydrated Salts
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Topics

Relative Atomic Mass, Relative Molecular Mass, and Carbon-12 Scale

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