Question

Washing soda is a hydrated compound whose formula can be written NA2CO3 xH2O, where x is the number of moles of H2O per mole of Na2CO3. When a 2.123g sample of washing soda was heated at 130*C, all of the water of hydration was lost, leaving the 0.786g of anhydrous sodium carbonate. Calculate the value of x

Answer 1

The value of X is 10 hence the of hydrated washing soda is Na2CO3.10H2O

calculation

Step 1: calculate the mass of water

= mass of hydrated washing soda - mass of anhydrous sodium carbonate ( anhydrous washing soda)

= 2.123 g - 0.786 g =1.337 grams

Step 2: calculate the moles of Na2CO3 and that of H2O using moles=mass/molar mass formula

• molar mass of Na2CO3= (2x23) + 12 + ( 16 x3) = 106 g/mol
• molar mass of H2O= (1 x2) +16= 18 g/mol

= moles of Na2CO3 therefore= 0.786 g/106 g/mol= 0.00742 moles

moles of H2O = 1.337 g/18 g/mol =0.0743 moles

Step 3: find the mole ratio

• that is divide each mole by smallest number of mole (0.00742)

= for Na2CO3 = 0.00742/0.00742 =1

for H2O= 0.0743/0.00742 = 10

hence the value of X =10