Answer
c.$$K_c = 1.2 \times 10^{-1}$$
Work Step by Step
1. Multiply all coefficients of the first reaction by 2, so find the value of Kc to the power of 2.
$$2HF(aq) \leftrightharpoons 2H^+(aq) + 2F^-(aq)$$
$$K_c = (6.8 \times 10^{-4})^2 = 4.6 \times 10^{-7}$$
2. Reverse the second reaction, so find the inverse of Kc.
$$2H^+(aq) + C_2{O_4}^{2-}(aq) \leftrightharpoons H_2C_2O_4(aq)$$
$$K_c = \frac{1}{3.8 \times 10^{-6}} = 2.6 \times 10^5$$
3. Thus:
$$2 HF(aq) + C_2O_4^{2-} \leftrightharpoons 2F^-(aq) + H_2C_2O_4(aq)$$
$$K_c = 4.6 \times 10^{-7} \times 2.6 \times 10^5 = 1.2 \times 10^{-1}$$
