Isn't Mn2+ also colourless?
Q.13. Which of the following is not coloured?
(1) M n 2 +
(2) C r 3 +
(3) Z n 2 +
(4) C u 2 +
Sol. Answer (3)

Z n 2 + does not show any colour because no any transition is possible there due to 3 d 10 configuration.

Dear student,
Mn2+ is having five unpaired electron in d orbital and hence can undergo d-d transition . So it may coloured .
But Zn​2+ is having ten electron in d orbital and hence completely filled d orbital . It cannot undergo d-d transition and hence is colourless.
Regards

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No Mn2+ has 5 unpaired electrons ie. partially filled d-orbital. Hence it is coloured. Where as Zn2+ has 3d10 configuration which is fully filled d-orbital and hence it is colourless. Colour will be shown only when there is partially filled d-orbital.
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But we use the colourless property of Mn2+ for the detection of SOright?
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What are you looking for?