Explain why stainless steel, sea water, and air are all considered solutions

They are composed by two or more pure substances.

Explanation:

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At first, we must take into account that mixtures are such materials made up of two or mire pure substances, say elements, molecules or compounds. They are classified as homogeneous for which there is no appreciable difference upon its constituents and as heterogeneous for which the constituents are differentiable. Now, stainless steel is considered as a mixture as long as it is mainly composed by iron and carbon and some other metallic elements such as chromium. Sea water is another interesting example, since the main constituents are water and sodium chloride, nonetheless, other ions such as iodide, sulfate, magnesium (II), potassium (I), calcium (II) are also found into it. Finally, the air is a mixture due to the presence of oxygen and nitrogen as the major constituents, considering others such as carbon dioxide, methane (pollution), argon, hydrogen as traces, it means, compositions below the 1%.

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n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.