Q42 of 65 Page 24

When a metal X is treated with cold water, it gives a basic salt Y with molecular formula XOH (Molecular mass = 40) and liberates a gas Z which easily catches fire. Identify X, Y and Z and also write the reaction involved.

X is Na , Y is NaOH and Z is H2


The reaction is : 2Na + 2H2O→ 2NaOH + H2 + Heat Energy


Since the molecular formula of Y = XOH, let’s suppose the atomic weight of metal X is a. Then, molecular mass of XOH = a + 16 + 1 = 400.


Then, a = 40 – 17 = 23. The atomic weight of sodium is 23, so metal X is sodium (Na). Sodium reacts with water to give hydrogen gas (Z) that catches fire.


So, metal X is sodium (Na), Y is (sodium hydroxide) and Z is H2 (hydrogen gas).


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