Practice problems/The Cyclops gets an IV drip

An IV drip is used to bring the Cyclops' Na⁺ ion levels back to normal (9g/L) from 5g/L.

• The drip rate is 0.1 L/hr.
• The Cyclops total blood volume is 2L.
• The Cyclops urinates 0.1 L/hr ensuring his total blood volume remains constant. His urine has a concentration that is exactly the same as his blood concentration at any moment.

Write down a differential equation for the the mass of Na⁺ in his blood as a function of time.