the mass of an electron is 9.1 ×10 power of -31 .if its k.e is 4×10 power of -25 j ,calculate the wave lenth

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Energy equals h(Planck's constant) times c(speed of light) divided by the wavelength. Therefore the wavelength will equal h(6.626 × 10power-34 J/ s) times c(2.998 x 10 power of 8 m/s) divided by E(4x10power of -25J). Joules and seconds cancel leaving you with meters. So the answer is approximately 0.4966 meters. You should check that with your calculator though.