One of my titration-related topic and I'm finding for a mystery akali metal element which has compound formula of M2CO3. M, being the mystery one (it can be any of the akali metals). From my data and calculation, I obtained an amu value of 31 g/mol. This value sits in the middle between Na and K (8 amus apart from each element). Which one should I reason my unknown element is. I can't decide whether it is more likely Na or K. Any help? Wht factors should I look at? I've rechecked my data over and over and I don't see to have errors in numbers unless my whole method is wrong.