How much salt do you add to an ounce of water to keep it from freezing?

Answer 1

Just to be clear: adding salt, or anything else, to water will not "keep it from freezing". They just make it freeze at a lower temperature.

The amount of salt you add depends on how much you need the freezing point to drop. If you just need to make sure it doesn't freeze at 0 degrees Celsius, then even a very small amount of salt will work. If you need it to stay liquid at -4 degrees Celsius, you're going to need to add a little over a mole of table salt per kilogram of water; a tenth of an ounce of salt per ounce of water should be more than sufficient.

You can only dissolve about six moles of table salt per kilogram of water, which corresponds to a freezing point of around -23 degrees Celsius. If the temperature is going to be below that, no amount of salt will keep the water from freezing.

Answer 2

To prevent water from freezing at 32°F, you can add approximately 3.5% salt by weight. For an ounce of water, this would equate to about 0.11 ounces (or 3 grams) of salt. However, the exact amount may vary depending on the temperatures you are trying to protect against.