[Mu] = 45;
[Sigma] = 12;
n = 15;
list = {};
Do[data = RandomVariate[NormalDistribution[[Mu], [Sigma]], n];
list = Append[list, Sqrt[Variance[data]]], 50000]
Show[Histogram[list, 100, "PDF"],
 Plot[P[n, [Sigma], s], {s, 0, 35}]]