# If the distance between the centres of the atoms of potassium and bromine in KBr (potassium – bromide) molecule…

Q: If the distance between the centres of the atoms of potassium and bromine in KBr (potassium – bromide) molecule is 0.282 ×10-9 m, find the centre of mass of this two particle system from potassium (mass of bromine = 80 u, and of potassium = 39 u)

Sol: Let position coordinate of potassium, xK = 0 xBr = 0.282 × 10-9 m.

∴ Co – ordinate of centre of mass.

$\Large x_{com} = \frac{m_K x_K + m_{Br} x_{Br}}{m_K + m_{Br}}$

$\Large x_{com} = \frac{39 \times 0 + 80 \times 0.282 \times 10^{-9}}{39 + 80}$

= 0.189 × 10-9 m