5 volume solution of H₂O₂ means that 1L of this H₂O₂ will give 10L of oxygen at STP

2H₂O₂(l) → O₂ (g) + H₂O(l)

2*34 g 22.4L at STP

68g

22.4L of O₂ at STP is produced from H₂O₂ = 68g

5L of O₂ at STP is produced from H₂O₂ = = 15.18g

Therefore, the strength of H₂O₂ in 5 volume H₂O₂ = 15.18g/L