Are there any differences between resolving power and dispersive power of grating?
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Resolving power defines how thin the peaks corresponding to the same wavelength will be. See e.g. this image of diffraction on 2 slits (top) vs 5 slits (bottom):
(source)
In this image, the top pattern is made by a higher-resolvance "grating" than the bottom one. The smaller the spot width, the finer difference between wavelengths you'll be able to detect.
Dispersive power, on the other hand, defines how far different diffraction orders will be from each other and how far different-wavelength peaks will be from each other in angular measure. In the image above dispersive powers of the "gratings" are the same: notice how each peak's center in the top pattern is above the corresponding peak in the bottom one.
