Volumes are not additive, check this paper:

"Densities and volume properties of (water + tert-butanol) over the temperature range of (274.15 to 348.15) K at pressure of 0.1 MPa"

doi:10.1016/j.jct.2010.10.018

of course you just add water to 100mg alcool up to 1L solution (or scale the quantities to your own needs). Density will be very close to 1, so consider if you really need additional accuracy

Hii..Sakti Sharmila..

As the density is 0.775g/ml = 775mg/ml

Required conc. 100mg/L = 100ng/ml

Then N1V1=N2V2

Say V2= 100ml then V1= (100ng*100ml)/775mg= 12.90ul

Add 12.90 ul in water to make final volume 100ml

Required concentration of t-butanol = Density x Volume of t-butanol/Volume of water

100 mg/L = 775 mg/mL*x/1 L

x = 100/775 = 0.129 mL or 129 microL.

Dissolve 0.129 mL of tr-butanol in 1 L of water gives you 100mg/l concentration.