Answer: 2.5 g/dm^3
Explanation: potassium manganate, KMnO4, has a molar mass of 197.1 g/mole. 5 g KMnO4 would be (5g/(197.1 g/mole) = 0.0254 moles K2MnO4.
Calculating moles was a waste of my time, becuse the answer requested is in 6/dm^3. But I'll leave it as a lesson to "always read the full question before answering." [Spoiler alert: I rarely do, and often waste my time].
We add 5g to 2 dm^3 of water. 5g/2dm^3 = 2.5 g/dm^3.
