What molarity of KMnO4 is needed if 10.0 L of it mixed with 7.5 L of water gives a 4.50 M KMnO4 solution?

Method 1 :

Before dilution : M₁ = ? M, V₁ = 10.0 L
After dilution : M₂ = (10 + 7.5) L = 17.5 L, V₂ = 4.5 M

M₁V₁ = M₂V₂
Molarity of KMnO₄ before dilution, M₁ = M₂ × (V₂/V₁) = 4.50 × (17.5/10.0) M = 7.875 M


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Method 2 :

No. of moles of KMnO₄ after dilution = (4.50 mol/L) × (10.0 + 7.5 L) = 78.75 mol
No. of moles of KMnO₄ before dilution = 78.75 mol
Molarity of KMnO₄ before dilution = (78.75 mol) / (10.0 L) = 7.875 M