According to the equation 1 mole of Cl2 is formed ffrom 1 mole of MnO2 

ie 22.4 L of Cl2 is formed from 87 g of MnO2 
 hence 2.24 L will be formed from 8.7 g of MnO2 
hence impurity in MnO2 is = 10 - 8.7 g =  1.3 g 
hence % impurity = 1.3  / 10  x 100 = 13%

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