2X + 2HCl  → 2XCl  +  H$_2$ In the equation above, X is

Analyze the question

The equation is 2X + 2HCl → 2XCl + H₂. The product is XCl, so X forms a chloride with a 1:1 ratio with chlorine. Since Cl⁻ has a charge of 1−, X must form an X⁺ ion, meaning X is a Group 1 metal (valency 1).

Concept

The formula of a metal chloride depends on the metal's valency:

• Group 1 metals (Na, K, Li) form M⁺, giving MCl.
• Group 2 metals (Mg, Ca, Ba) form M²⁺, giving MCl₂.

Check each option

• A. K: K⁺ gives KCl. The equation 2K + 2HCl → 2KCl + H₂ is balanced (2 K, 2 H, 2 Cl on each side). ✔
• B. Mg: gives MgCl₂, and the balanced equation is Mg + 2HCl → MgCl₂ + H₂. ✘
• C. Ca: gives CaCl₂, and the balanced equation is Ca + 2HCl → CaCl₂ + H₂. ✘
• D. Ba: gives BaCl₂, and the balanced equation is Ba + 2HCl → BaCl₂ + H₂. ✘

Answer

Only potassium (K) is monovalent among the options, so it fits the formula XCl.

(In practice, potassium reacts explosively with acid, but the question only tests valency and balancing.)

Tip: In questions like this, look at the formula of the product. If it is XCl, the metal is Group 1. If it is XCl₂, the metal is Group 2. Remember that Group 1 = valency 1, Group 2 = valency 2.