If the atomic number of an element is 6 and its mass number is 15, how many neutrons are in the atom's nucleus?

Atomic Number is the same as the number of protons in an element.
Mass Number is the number of Protons + Neutrons in an element.

Atomic Number: 6 means 6 Protons
Mass Number: 15 means 15 atoms that are a proton/neutron.
We are given out of the 15 atoms, 6 of them are protons, so the other 9 must be Neutrons.

15 - 6 = 9 so there must be 9 Neutrons.

There are 9 Neutrons in atom's nucleus.

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boiling point is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. the vapor pressure of solvent molecules is decreased when a solute is added, so a higher temperature is required to increase the number of solvent molecules in the gas phase above the liquid.

at the freezing point, the vapor pressures of the solid and liquid are equal; a lower temperature is needed to reduce the number of solvent particles above the liquid.

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tobey