Is potassium manganate the same as potassium permanganate?
Also, I have seen potassium manganate written as potassium manganate (VII) or potassium manganate (VI).
What do the roman numerals stand for?
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1$\begingroup$ The roman numerals describe the oxidation state of the manganese. See chemistry.stackexchange.com/questions/62333 and chemistry.stackexchange.com/questions/59777 for other examples. $\endgroup$– Klaus-Dieter WarzechaJan 13, 2017 at 12:41
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1$\begingroup$ I'm voting to close this question as off-topic because of what Klaus said. $\endgroup$– Todd MinehardtJan 14, 2017 at 2:16
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2 Answers
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No, they are somewhat different. $\ce{K2MnO4}$ is potassium manganate and $\ce{KMnO4}$ is potassium permanganate. Notice the two potassium for each $\ce{MnO4}$.
The Roman numerals mark the oxydation state. So, manganate will have a (VI) and permanganate will have a (VII).
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There are two different nomenclature concepts at work here. One includes potassium manganate and potassium permanganate and the other uses potassium manganate(VII) and potassium manganate(VI). These two do not mix, and each of the two has an unambiguous name for each compound. Thus, to answer your title question, potassium permanganate and potassium manganate are not the same.
The more easily explained difference is that between manganate(VI) and manganate(VII) (the cation is irrelevant; as always). The Roman numerals represent oxidation states as they always do. In both cases, we are dealing with $\ce{MnO4^n-}$ and the oxidation state numeral tells us the exact charge. If you do your maths correctly, you will arrive at the following:
$$\begin{align}\text{manganate(VI)} && \ce{\overset{{+VI}}{Mn}\overset{-II}{O}_4^n-} & \Longrightarrow &&\ce{MnO4^2-}&& \Longrightarrow &\ce{K2MnO4}\\ \text{manganate(VII)} && \ce{\overset{{+VII}}{Mn}\overset{-II}{O}_4^n-} & \Longrightarrow &&\ce{MnO4-}&& \Longrightarrow &\ce{KMnO4}\end{align}$$
The two substances differ by the number of potassium counterions present in the crystal; potassium manganate(VI) has two per manganate(VI) anion while potassium manganate(VII) has one.
For reasons not entirely known to me, it was decided once that manganate(VI) — $\ce{MnO4^2-}$ — should be the ‘standard manganate’. It fits in well if the analogy to sulphate ($\ce{SO4^2-}$) and chromate ($\ce{CrO4^2-}$) which all have the $\mathrm{+VI}$ oxidation state and same charge; but that’s the last difference. -ate names are not commonly used for the highest oxidation state only (e.g. chlorate $\ce{ClO3-}$) and they do not refer to a certain oxidation state (e.g. phosphate with the oxidation state $\mathrm{+V}$). Whatever the reasons were, manganate was decided to be $\ce{MnO4^2-}$, i.e. the manganate(VI) you just heard about above.
The other ‘manganate’-like anion, manganate(VII) from above, is in a higher oxidation state. At least this part of the nomenclature is consistent: the higher oxidation state is marked by the prefix per. Thus, permanganate must have a higher oxidation state than manganate.
Summary: Manganate always maps to manganate(VI) and vice-versa; permanganate always maps to manganate(VII) and vice-versa.
