Question

Under the correct conditions, the element molybdenum emits X-rays with a characteristic wavelength of 0.711A (7.11*10^-11m). These X-rays are used in diffraction experiments to determine molecular structures. How fast does an electron need to be moving to have the same wavelength as these X-rays?

Wavelength=h/mv

7.11*10^-11m= (6.626*10^-34J*s) / mv

I'm stuck. How do I know what "m" is? Is it just the atomic mass of Molybdenum, 95.9amu?
No. Aren't you interested in the speed of the electron. So it's the mass of the electron (in kg I think).
Oh, yes that's right. Mass of electron= 9.11*10^-31kg.

Therefore v=1.02*10^7m/s
That looks ok to me. I didn't check the arithmetic.

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