Place 200 ml of water in a small bowl, and stir in the citric acid until fully dissolved. In another small bowl, add in the remaining 50 ml of water and stir in the rennet until fully dissolved.
Pour the milk in a large pot, and stir in the citric acid mixture. Place the pot over medium-low heat and stir the milk gently until reaching 90°F/32°C, about 5 minutes.
Remove the pot from the heat and slowly stir in the rennet mixture.
Continue to stir for 30 seconds, then cover the pot with a lid and let it sit for 5 minutes.
Remove the lid from the pot and make sure the milk has reached a tofu-like consistency (if not, allow to set an extra 10 minutes). Use a sharp knife to cut the curds into horizontal and vertical lines, creating a grid-like pattern. Make sure to cut the lines all the way through the bottom of the pan.
Return the pot to medium-low heat and stir the curds gently until reaching 110°F/43°C. Remove the pot from the heat and continue to stir the curds slowly for 5 more minutes.
With the help of a strainer (or slotted spoon), remove the curds from the pot and place them in a large bowl. Reserve 2-3 cups of the whey to preserve the mozzarella later.
Pour enough hot water to cover the curds into the bowl, and allow them to sit for 2-3 minutes, until reaching 135°F/57°C. Sprinkle the salt over the curds and quickly work the curds, making sure to wear rubber gloves.
Use both hands to stretch and fold the curds a few times, until you have a firm shiny mozzarella “dough”. If it still has a cottage-cheese consistency, drain off the water, add more hot water and continue to stretch and knead the cheese until needed. Be careful not to overwork the dough, or your mozzarella will end up having a rubbery consistency.
Once your mozzarella dough is ready, shape into a large mozzarella ball or make smaller balls, and place them in an air-tight container.
Mix the reserved whey with a generous pinch of salt and pour it over the mozzarella balls. Eat as soon as it cools down, or store the cheese balls in their water in the fridge for up to 2 days.