A mixture of 1.65*10²¹ molecules of X and 1.85*10²¹ molecules of Y weighs 0.688 g. The mol.wt. of X (assume mol.wt.of Y is 187)?

Here, we have,

187 gm of Y = 1 mole of Y = 6.022 x 10²³ molecules of Y

So, if  6.022 x 10²³ molecules of Y = 187 g

then, 1.85 x 10²¹ molecules of Y = (187 /  6.022 x 10²³ ) x 1.85 x 10²¹  = 0.574 gm of Y

Hence, mass of 1.65 x 10²¹ molecules of X = 0.688 -0.574 = 0.114 gm

Since, mass of 1.65 x 10²¹ molecules of X = 0.114 gm

So, mass of 6.022 x 10²³ molecules of X = (0.114/ 1.65 x 10²¹ ) x 6.022 x 10²³ =  41.6 gm = mass of 1 mole of X

Hence, molar mass of X = 41.6 gm