lb? Are you serious? Trick question, perhaps?
Do you mean 1.41 kg or shall we leave the answer up to gravity?
molar mass ???, all you need is to ratio of grams in a pound
1 = x grams / pound
3.1 lb * x grams / pound = 3.1x grams of titanium
divide by densitivy of titanium to give u cm^3 and then convert again to cubic inches