Solution to Self test Questions on water of Crystallization.

 Hello fam! Due to some technical issues we could not post our lesson yesterday. However, today, we are going to consider the solutions to the self test problems that we gave on Tuesday. This is to help everyone to understand the concept better.


Self test problems;

1) Determine the value of X in the formula of washing soda crystals given the equation below;

Na2CO3.xH2O -> Na2CO3 + xH2O

28.6g                         10.6 g

2) A 46.1 g sample of hydrated potassium dichromate K2Cr2O7.xH2O was heated to give 35.3 g of anhydrous salt. What is the value of x?

Solution;

Question 1;

From the question;

Mass of hydrated salt = 28.6g  

Molar mass of hydrated salt = 2(23) + 12 + 3(16) + x[2(1) + 16]

106 + 18x

Mass of anhydrous salt =  10.6 g

Molar mass of anhydrous salt = 2(23) + 12 + 3(16)

= 106 g/mol


Again;

Number of moles of hydrated salt = Number of moles of anhydrous salt

But number of moles = mass / molar mass

Number of moles of hydrated salt = 28.6 g/106 + 18x g/mol

Number of moles of hydrated salt = 10.6 g/106 g/mol

28.6/106 + 18x = 10.6/106 

106 * 28.6 = 10.6 (106 + 18x)

3031.6 = 1123.6 + 190.8x

3031.6 – 1123.6 = 190.8x

x = 10

The formula of the compound is Na2CO3.10H2O


Question 2

Mass of hydrated salt = 46.1 g

 Molar mass of the hydrated salt = 2(39) + 2(52) + 7(16) +  x[2(1) + 16]

= 78 + 104 + 112 + 18x

= 294 + 18x g/mol

Number of moles of hydrated salt = 46.1 g/294 + 18x g/mol

Mass of anhydrous salt = 35.3 g 

Molar mass of  anhydrous salt = 2(39) + 2(52) + 7(16)

= 294 g/mol

Number of moles of anhydrous salt = 35.3 g/294 g/mol


Then;

Number of moles of hydrated salt = Number of moles of anhydrous salt

  46.1 /294 + 18x = 35.3 /294 

46.1 * 294 = 35.3(294 + 18x)

13553.4 = 10378.2 + 635.4x

13553.4 - 10378.2 = 635.4x

x = 5

The formula of the compound is K2Cr2O7.5H2O


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