Question

at 298 k ph of 0.1m weak acid is 5.37.calculate ionisation constant of weak acid

Answer 1

You need to use capitals correctly. M stands for molarity = moles/L of solution. m stnads for molality = moles/kg solvent. I will assume you mean M = 0.1 M
pH = 5.37 = -log (H^+)
(H^+) = 4.27E-6
..........................HA --> H^+ + A^-
I.........................0.1M......0.......0
C........................-x...........x.......x
E..................0.1-x............x........x
Ka = (H^+)(A^-)/(HA)
Solve for x, evaluate 0.1-x then substitute those values into the Ka expression and solve for Ka. Post your work if you get stuck.