Not my words but this gives a good explanation...
Tender Quick is not the same as prague. It is a proprietary blend and contains salt, sugar, both sodium nitrate and sodium nitrite (either or both cause the characteristic red color we associate with cured meats, markedly extend shelf life, and allow a much greater margin of safety (by inhibiting bacteria) for cold smoking or allow for the curing of meats meant to be consumed uncooked) and propylene glycol to keep the mixture uniform. It cannot be used as a 1:1 exchange with prague powders.
Prague #1 (Insta-Cure #1, Modern Cure, DC #1, DQ #1, et. al., all the '#1's) is 6.25% sodium nitrite and 93.75% salt. It's used in dry cures and in brine cures, primarily for meat that will be smoked (though not necessarily) and cooked for service. It's used at the rate of 1oz/25lbs meat or 1 level teaspoon/5lbs meat.
Prague #2 (Insta-Cure #s, et. al., all the '#2's) is for dry-cured meats that require long aging (weeks or months) to cure and/or develop flavors and for products that do not require cooking, smoking, or refrigeration. It contains 1oz of sodium nitrite and .64oz sodium nitrate per pound of salt. Sodium nitrate acts as sort of a time-release cure and slowly breaks sown into sodium nitrite and then into nitric acid. It is also used at the rate of 1oz/25lbs meat or 1 level teaspoon/5lbs meat.
Some #1 and #2 products are colored pink so that they can easily be distinguished from plain salt because large amounts of nitrite/nitrate can be dangerous. If you look at the figures above, very little nitrite or nitrate is needed per pound of meat to achieve the best cure, shelf life, and reduction of potential bacteriological activity. Just 6.25% of that single level teaspoon is sodium nitrite and this is enough for 5lbs of meat. This is why prague powders were developed in the first place. Because the amount of chemical(s) needed is so small manufacturers pre-mixed salt with the appropriate chemical(s) in the proper balance so that they could be more easily and correctly measured.
It is important to have a good understanding of curing products before you use them. Follow manufacturer's recommendations and be careful measuring and mixing. Reducing or increasing meat amounts for a given recipe require like reduction or increase in cure mixes.