The coefficient ya in a balanced chemical equation always can express the ratio of ?

The answer is: moles of reactants and products.

For example, balanced chemical equation:

4NH₃(g) + 6NO(g) → 5N₂(g) + 6H₂O(l).

1) Mole ratio 1: n(NH₃) : n(NO) = 4 : 6 (2 : 3).

2) Mole ratio 2: n(NH₃) : n(N₂) = 4 : 5.

3) Mole ratio 3: n(NH₃) : n(H₂O) = 4 : 6 (2 : 3).

4) Mole ratio 4: n(NO) : n(N₂) = 6 : 5.

5) Mole ratio 5: n(NO) : n(H₂O) = 6 : 6 (1 :1).

6) Mole ratio 6: n(N₂) : n(H₂O) = 5 : 6.