Usually yes. Some recipes specify "baking" cocoa powder just so that nobody gets confused and uses "drinking chocolate mix" instead (since this contains sugar and milk powder which would throw off the recipe).
Baking soda is required to leaven the baked product. Either baking soda or baking powder would need to be included in the batter regardless of the use of cocoa powder.
Mealted Chocolate
Simple cocoa powder does not contain any dairy products, but only powdered, processed cocoa (cacao) beans. However, there are many commercial products with some version of "cocoa" in the name, such as "Hot Cocoa," "Cocoa Mix," or "Instant Cocoa" that do contain a high percentage of dairy. The safe thing to do is to read the ingredients listed on the product's label.
One cup of dry cocoa is about 150g so 75g would be half a cup.
Cocoa is made by separating the cocoa butter out of chocolate liquor, then drying the defatted chocolate liquor. You could probably mix sugar, cocoa powder and cocoa butter into a chocolate, but that would be more trouble than it's worth.
One way to separate cocoa powder from water is through filtration. Place the cocoa powder and water mixture in a filter, such as a coffee filter or cheesecloth, and allow the water to pass through while retaining the cocoa powder. Another method could be to let the mixture settle, allowing the cocoa powder to sink to the bottom, and then carefully decanting the water from the top.
The number of cups in 500 grams of cocoa powder can vary slightly depending on the density of the cocoa. Generally, cocoa powder weighs about 100 grams per cup, so 500 grams would be approximately 5 cups. However, it's best to use a kitchen scale for precise measurements in baking.
You can't unless they are melted. If you need to melt chocolate chips, you need to use cocoa powder, butter and sugar. Every ounce of chocolate can be replaced by 3 TBLS of Cocoa Powder (unsweetened) and 1 TBL of Butter and 3 TBLS of Sugar
Unsweetened cocoa powder contains a significant amount of carbohydrates, protein, and fiber, so it would be placed in the category of macromolecules known as carbohydrates and proteins on a macromolecule chart. It doesn't contain fat or cholesterol.
An object with the least mass and velocity would have the least momentum.
To substitute cocoa powder for baker's chocolate, you would use 4 tablespoons of cocoa powder for every 1 ounce of baker's chocolate. Therefore, to replace 4 ounces of baker's chocolate, you would need 16 tablespoons of cocoa powder. Keep in mind that cocoa powder is less fatty than baker's chocolate, so you may need to adjust the recipe by adding a bit more fat, such as butter or oil, to compensate for the difference.