The depression in freezing point observed for a formic acid...

$\left[HCOOH\right] = 0 . 5ml{L}^{- 1}$

$⇒ \left(0 . 5ml \times 1 . 05g{ml}^{- 1}\right)HCOOH$ in 1L

$⇒ 0 . 525gHCOOH in 1L$

Weight of water will be $= 1kgsincedensityofwateris1kg / L$

$m = \dfrac{\left(\dfrac{0 . 525}{46}\right)}{1kg}mol$ [Assuming dilute solution]

$∴ {\Delta T}_{f} = {iK}_{f}m ⇒ i = \dfrac{{\Delta T}_{f}}{{k}_{f}m} = \dfrac{0 . 0405 \times 46}{1 . 86 \times 0 . 525} = 1 . 9$