Is zakah due on the 4.5 lakhs in cash that my mother has?

My dad passed away in the year Jan 2012 and he was a government servant, so we received his dues from Govt. in the month of November 2012 in total 10 lakh rupees. The money came in the name of my mother brother and me, with this we paid his loans and the loans which he had taken on our behalf for our education. Myself and my brother willing gave the ownership of the remaining money to our mother 6.5 lakhs. Now with this money we purchased a land worth 7.2 lakhs in mother’s name, 3 lakhs already paid and remaining 4.2 lakhs have to be paid after registration of the land (date is not fixed). The intention of purchasing the land was to avoid keeping the money in bank to avoid the interest. If any good society develops there we cloud build on the land, give for rent and also if needed sell the land. Now my mother is having total of 4.5 lakh cash in hand. What is the ruling of zakah in this case?