HCl + MnO2 → MnCl2 + H2O + Cl2 To balance this reaction, we have to have an exact number of electrons of each element on both sides. So it will turn into:
4HCl + MnO2 → MnCl2 + 2H2O + Cl2
From that, we could see that the coefficient for the HCL is 4.
