The amount of water (density 1.00 g mL-1) in grams that must be added to 26.2 g of MgCl2 in the preparation of a 1.5 % by mass solution is:​

1720.8g water are necessaries

Explanation:

Mass percent is defined as the mass of solute (In this case, MgCl2) in 100g of solution (Mass MgCl2 + Mass water). To solve this question we must find the mass of solution that we need to produce th 1.5% by mass solution. Thus, we can find the mass of water that we need as follows:

Mass solution:

26.2g MgCl2 * (100g Solution / 1.5g MgCl2) = 1747g solution

Mass water:

1747g solution - 26.2g MgCl2 = 1720.8g water are necessaries

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.