[rabolisk]

The mass of the unknown liquid is not 88.42g, although you will need that value (88.42) to find the mass of the unknown liquid. For the volume of the unknown liquid, again, if you have a cup with a certain volume, and you fill it with some liquid, then their volumes have to be the same. After all, in the real world, this is how you would measure volume of a liquid, using a container with a known volume (like a bucket that can hold a gallon, or a graduated cylinder with marks for volume).

[lokobreed]

ok so would i take mass of container with liquid 88.42 - mass of empty container 78.91?

im sorry as similaar as these equations are im so confused on this one

[rabolisk]

That's right. Also, volume of container = volume of unknown liquid (justify this to yourself).

[lokobreed]

ok so since the density of water is 1.0000 g/cm^3 then the volume of the container of a mass of 78.91 g when empty would be 78.91cm^3 right?

So the mass of container fill with liquid would have a volume of 9.51 g/cm^3?

[rabolisk]

No. I'm going to solve this problem for you so you can see what I'm doing.

A container has a mass of 78.91 g when empty and 103.782 g one filled with water.  The density of water is 1.0000 g/cm3.  Calculate the volume of the container.  When filled with an unknown liquid, the container had a mass of 88.42 g.  Calculate the density of the unknown liquid.

Part 1
Volume of the container = volume of water (when water fills up the container)
Volume of water = Mass_H2O / density_H2O
Mass_container + mass_H2O = total mass
Mass_H2O = total mass - mass_container
Volume_H2O  = (total mass - mass_container) / density_H2O
Volume_container = volume_H2O = (103.782 - 78.91g)/(1.0000g/cm³) = 24.872 cm³ = 24.87 cm³ (4 sig figs)

From now on, I'm going to use the Greek letter rho (ρ) to denote density, m to denote mass, and V to denote volume.

Part 2
Total m = m_container + m_liquid
V_liquid = V_container = 24.872 cm³
ρ_liquid = m_liquid / V_liquid = (total m - m_container) / (V_liquid) = (88.42 - 78.91g)/(24.872cm³) = 0.3824 g/cm³ (4 sig figs)

Note that I used variables (e.g. V_container) rather than the actual values until the very end. This minimizes any calculation error, and rounding error.