Which of the following chemical equations violates the law of conservation of mass?

• A. Na + Cl_2 → 2NaCl
• B. 2K + 2H_2O → 2KOH + H_2
• C. C_3H_8 + 5O_2 → 3CO_2 + 4H_2O
• D. 2H_2 + O_2 → 2H_2O

Answer: (A)

Verified step by step guidance

1. Recall that the law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides of the equation.
2. Examine the first equation: \(\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}\). Count the atoms on each side. On the left, there is 1 Na atom and 2 Cl atoms. On the right, there are 2 Na atoms and 2 Cl atoms. Notice the sodium atoms are not balanced.
3. Check the second equation: \$2\text{K} + 2\text{H}_2\text{O} \rightarrow 2\text{KOH} + \text{H}_2$. Count atoms on both sides to verify if they are balanced.
4. Analyze the third equation: \(\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}\). Count carbon, hydrogen, and oxygen atoms on both sides to confirm balance.
5. Review the fourth equation: \$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$. Count hydrogen and oxygen atoms on both sides to ensure the equation is balanced.