To solve this problem, we will first find the mass of the paraffin, then use the relative density of paraffin to find the mass of an equal volume of water, and finally calculate the total mass of the bottle when filled with water. Step 1: Calculate the mass of the paraffin. The mass of the empty bottle is $30 \text{ g}$. The mass of the bottle filled with paraffin is $70 \text{ g}$. The mass of the paraffin ($m_{\text{paraffin}}$) is the difference between these two masses: $$m_{\text{paraffin}} = m_{\text{bottle+paraffin}} - m_{\text{empty}}$$ $$m_{\text{paraffin}} = 70 \text{ g} - 30 \text{ g}$$ $$m_{\text{paraffin}} = 40 \text{ g}$$ Step 2: Use the relative density of paraffin to find the mass of an equal volume of water. Relative density (RD) is defined as the ratio of the density of a substance to the density of water. For equal volumes, it is also the ratio of the mass of the substance to the mass of water. $$\text{RD}_{\text{paraffin}} = \frac{m_{\text{paraffin}}}{m_{\text{water (same volume)}}}$$ We are given $\text{RD}_{\text{paraffin}} = 0.8$ and we found $m_{\text{paraffin}} = 40 \text{ g}$. We need to find $m_{\text{water (same volume)}}$. $$0.8 = \frac{40 \text{ g}}{m_{\text{water (same volume)}}}$$ Rearrange the formula to solve for $m_{\text{water (same volume)}}$: $$m_{\text{water (same volume)}} = \frac{40 \text{ g}}{0.8}$$ $$m_{\text{water (same volume)}} = 50 \text{ g}$$ Step 3: Calculate the mass of the bottle when filled with water. The mass of the bottle filled with water ($m_{\text{bottle+water}}$) is the sum of the mass of the empty bottle and the mass of the water that fills it: $$m_{\text{bottle+water}} = m_{\text{empty}} + m_{\text{water (same volume)}}$$ $$m_{\text{bottle+water}} = 30 \text{ g} + 50 \text{ g}$$ $$m_{\text{bottle+water}} = 80 \text{ g}$$ The final answer is $\boxed{\text{80 g}}$.