How can I convert the number of particles per liter to gram per liter

How can I convert the number of particles per liter to gram per liter? I know the size of these particle and so, I calculated the volume and then by the density I calculate the mass. After that I don't know what to do

• Do you know which substance this is or only how many particles per liter are in your solution? – pH13 - Yet another Philipp Jul 14 '15 at 12:09
• I am not sure about the substances. Because I am still investigating that. They are about 8e8 particles per ml – Medi Jul 14 '15 at 12:22

$$x \frac{particles}{L} \times \frac {1 mol}{6.023 \times 10^{23}particles}\times \frac{MW gm}{mol}$$