Weighted Standard Deviation in Excel

Question

I have a dataset in excel that looks like this:

Impressions Clicks  CTR
          2      2    1
          5      4  0.8
         14      5  0.357142857
     2481574  2946  0.00118715
     4835785  7453  0.001541218
     6224189 10903  0.001751714
     5364316  7595  0.001415838
     4445746  7003  0.001575214
     3789942  3881  0.001024026
     3015085  3290  0.00109118
     2795202  2823  0.001009945
     2727213  2976  0.001091224
     2770730  3318  0.001197518
     2770752  3932  0.001419109
     2467906  3515  0.001424284
     1842635  3470  0.001883173
     1714929  3833  0.002235078
     1642027  3808  0.002319085
     1688948  4123  0.002441165
     2210116  5298  0.002397159
     1758368  4485  0.002550661
       48611    55  0.001131431
           1     0  0
           1     0  0
           1     0  0
           1     0  0
           1     0  0
           5     0  0
           2     0  0
           1     0  0

I want to determine the weighted standard deviation of the CTR column In Excel. For example there are examples of CTR being 0 but that's because the number of impressions is only 1 so it really doesn't justify the variance let alone the mean.

Does anyone know a formula in excel that can do this? I am looking specifically for an Excel formula not just a mathematical formula.