How to calculate the oscillator capacitor value for a switched capacitor (MAX7405)?
For 50 Hz cut-off frequency, you would need fOSC = 5000 Hz = 5 kHz, because it says on the data sheet that, there is a 1:100 ratio from corner frequency (or cut-off frequency) to internal oscillator frequency fOSC. Then the equation requires the numerical value of fOSC in kHz units, which we will use fOSC(kHz) = 5 for this. Then K = 34. Our equation will read as:

fOSC( kHz) = (K * 10^3)/ Cosc; Cosc in pF

5          = (34* 10^3)/ Cosc;

Cosc = 6800 solving this equation. 

But remember that this value is given in pF. So this means that Cosc = 6800 pF = 6.8 nF. I think you only had a confusion about the (C*1 pF) part, where you could better write (C/1 pF) for the correct unit conversion.