To calculate the density of the liquid, you can use the formula for density:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
In this case, the mass of the liquid is 6 grams, and the volume is 12.0 milliliters. Now, substitute these values into the formula:
\[ \text{Density} = \frac{6 \text{ g}}{12.0 \text{ ml}} \]
Now, perform the division:
\[ \text{Density} = 0.5 \text{ g/ml} \]
Thus, the density of the unknown liquid is 0.5 g/ml.