Captial Gains is the difference between the sale and purchase price. In case of inheritance the purchase price becomes the cost of the property for your mother and its transfers accordingly.
Now when the asset is transferred, it is declared with purchase value (in your case, your mothers cost for the property). As soon as you sell the property, the difference between your declared value and sale price is known as capital gains and will be taxable.
You said the purchaser withheld 11% taxes, although these rates are found nowhere and need explanations further, but if we assume your status was active filer, then 4.5% and for late filers 7.5% while in case of non-filer this is 11.5% for properties below 50 million. These taxes can be adjusted against the capital gains tax as explained above.
236C advance tax on property as mentioned in 2nd para, cannot be adjusted against CGT if the property sold was acquired in the same tax-year. This is not your case as your acquiring and selling year (financial) are different.