The Reaction

You haven't provided the chemical reaction, but I'm going to assume it's:

```

2 H₂O₂ (aq) ⇌ 2 H₂O (l) + O₂ (g)

```

Equilibrium Constant (K)

The equilibrium constant (K) for this reaction is 0.62. This tells us the ratio of products to reactants at equilibrium.

Given Concentrations

You've given the following concentrations at equilibrium:

* [O₂] = 0.40 M

* [H₂O] = 0.20 M

Finding [H₂O₂]

We can use the equilibrium constant expression to find the equilibrium concentration of H₂O₂:

```

K = [H₂O]²[O₂] / [H₂O₂]²

```

Let's plug in the known values and solve for [H₂O₂]:

```

0.62 = (0.20)²(0.40) / [H₂O₂]²

```

Now, solve for [H₂O₂]:

1. Simplify: 0.62 = 0.016 / [H₂O₂]²

2. Rearrange: [H₂O₂]² = 0.016 / 0.62

3. Calculate: [H₂O₂]² ≈ 0.0258

4. Take the square root: [H₂O₂] ≈ 0.16 M

Therefore, the equilibrium concentration of H₂O₂ is approximately 0.16 M.