### Question 2.52

NCERT Class XI Chemistry

Following results are observed when sodium metal is irradiated with different wavelengths. Calculate (a) threshold wavelength and, (b) Planck's constant. λ (nm) 500 450 400 v × 10-5 (cm s-1) 2.55 4.35 5.35

Note: Q 2.44 to 2.51 are simple enough. So they have been skipped. Please write an email to us if you face any difficulty with any of them.### Solution in Detail

(video solution below this)

$\displaystyle \underline{\underline{\text{Threshold Wavelength}}}$

Plot the graph as shown in the video to obtain threshold wavelength $\displaystyle \lambda_0 = 530.9 \text{ nm}$

$\displaystyle \underline{\underline{\text{Planck's Constant}}}$

by photoelectric equation

$\displaystyle \frac {hc}{\lambda} = \frac {hc}{\lambda_0} + 1/2\: m_e\:v^2$

Put $\displaystyle \text{(1)} \lambda = 450 \times 10^{-9} \text{ m}$

$\displaystyle \text{(2)} v = 4.35 \times 10^{-7} \text{ m/s}$

$\displaystyle \text{(3)} \lambda_0 = 530.9 \times 10^{-9} \text{ m}$

$\displaystyle \text{(4)} c = 3 \times 10^{8} \text{ m/s}$

$\displaystyle \text{(5)} m_e = 9.1 \times 10^{-31} \text{ kg}$

Solving, $\displaystyle h = 6.15 \times 10^{-34}\text{ Js }\underline{Ans}$

### Video Explanation

Please watch this youtube video for a quick explanation of the solution:

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