It's useful because it highlights structures in biological tissue.
Answer:

Explanation:
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In this case, for such unit conversion we need to realize that 1 kg is equal to 1000 g, 1 g is equal to 1x10⁹ and 1 mL equals 1 cm³, therefore we apply:

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The type of ion that the sodium atom form is a cation. The term cation is used to refer to ions which have positive charges. The valence electron of sodium is one and in order to obtain the octet, it will donate that electron to some other atoms, making its charge positive.
2.50 x 6.02 x 10^23 = 1.51 x 10^24 molecules (A)