A hydrate of CoCl2 with a mass of 6.00 g is heated strongly. After cooling, the mass of the anhydrate is 3.27 g.
a. How many grams of H2O were lost from the hydrate?
b. What is the % water in the hydrate?
c. What is the formula of the CoCl2 hydrate?
d. Write the equation for the dehydration of CoCl2 hydrate.
The correct answer and explanation is:
Sure! Let’s analyze the problem step-by-step.
Given:
- Mass of hydrate (CoCl₂·xH₂O) = 6.00 g
- Mass of anhydrate (CoCl₂) after heating = 3.27 g
a. How many grams of H₂O were lost from the hydrate?
The water lost = Mass of hydrate − Mass of anhydrate
= 6.00 g − 3.27 g = 2.73 g
b. What is the % water in the hydrate?
% water = (mass of water / mass of hydrate) × 100
= (2.73 g / 6.00 g) × 100 = 45.5%
c. What is the formula of the CoCl₂ hydrate?
We need to find the number of water molecules, x, in CoCl₂·xH₂O.
- Calculate moles of anhydrate (CoCl₂):
- Molar mass of CoCl₂ =
Co = 58.93 g/mol
Cl = 35.45 g/mol × 2 = 70.90 g/mol
Total = 58.93 + 70.90 = 129.83 g/mol
Moles CoCl₂ = mass / molar mass = 3.27 g / 129.83 g/mol ≈ 0.02517 mol
- Calculate moles of water lost:
- Molar mass of H₂O = 18.02 g/mol
Moles H₂O = 2.73 g / 18.02 g/mol ≈ 0.1515 mol
- Find mole ratio of H₂O to CoCl₂:
Ratio = moles H₂O / moles CoCl₂
= 0.1515 / 0.02517 ≈ 6.02 ≈ 6
Thus, the formula of the hydrate is CoCl₂·6H₂O.
d. Write the equation for the dehydration of CoCl₂ hydrate.
When the hydrate is heated, it loses water and forms anhydrous cobalt(II) chloride: CoCl2⋅6H2O→heatCoCl2+6H2O\mathrm{CoCl_2 \cdot 6H_2O \xrightarrow{\text{heat}} CoCl_2 + 6H_2O}
Explanation (300 words):
Hydrates are compounds that contain water molecules integrated into their crystalline structure. Heating a hydrate usually causes the water molecules to evaporate, leaving behind the anhydrous salt.
In this problem, a 6.00 g sample of cobalt(II) chloride hydrate (CoCl₂·xH₂O) is heated until all the water is lost, leaving 3.27 g of anhydrous CoCl₂. The difference in mass (6.00 g − 3.27 g = 2.73 g) represents the mass of water lost.
To determine the water percentage in the hydrate, the mass of water lost is divided by the original hydrate mass and multiplied by 100, giving 45.5%. This means nearly half of the mass of the hydrate is due to water.
Next, to find the exact number of water molecules per formula unit (the value of x), we convert the masses to moles. The moles of anhydrous CoCl₂ are calculated using its molar mass (129.83 g/mol), and the moles of water lost are calculated using water’s molar mass (18.02 g/mol). The ratio of moles of water to moles of CoCl₂ comes out to approximately 6, indicating that each formula unit of cobalt(II) chloride hydrate contains 6 water molecules.
Finally, the dehydration reaction can be written as: CoCl2⋅6H2O→heatCoCl2+6H2O\mathrm{CoCl_2 \cdot 6H_2O \xrightarrow{\text{heat}} CoCl_2 + 6H_2O}
This reaction demonstrates the loss of water molecules upon heating, transforming the hydrate into its anhydrous form.