Ammonium acetate is soluble in plain water up to something around 18 M. It should then completely dissolve for a 10 M solution. Increase temperature to accelerate the dissolution, use a close vessel and do not open it until cooling the solution down.

You may need to make a larger volume of your 10M ammonium acetate. It might be easier to make 500mL than 70mL. Another thought, what water are you using? Is it ultra pure and straight from the filter? If the water has been sitting for a while the pH may have gone basic and hinder dissolving the solute.

It is a white, hygroscopic solid and can be derived from the reaction of ammonia and acetic acid.

IUPAC name ammonium ethanoate

Chemical formula NH4CH3CO2

Molar mass 77.083 g·mol−1

Ammonium acetate is soluble in water at variable quantities.

Solubility in water

102 g/100 mL (0 °C) 148 g/100 mL (4 °C)[1] 143 g/100 mL (20 °C) 533 g/100 mL (80 °C)

Density 1.17 g/cm3 (20 °C) 1.073 g/cm3 (25 °C)

So, If you wan to have have a 10 molar solution in water, you must dissolve 770,83 g/L = 77,083 g/100mL.

It is very soluble, so, I do not think it is necessary to heat.

Weigh the desired mass for volume of solution you wish; a liter (770,83g) or for 100 mL (77,083g) .

Put a few mililiters of water in the volumetric flask.

Add small quantities of solid ammonium acetate while you stir the flask.

Add a little bit more water alternating with a little bit more salt.

After having all salt inside the flask, fill the volumetric flask with water until the calibration line always stirring the solution inside.

It is an endothemic reaction, so, the flask will cool a little bit.

Thanks Omar, Dustin, Jorge and Ali for your suggestions. It dissolved when I raised the temperature to 50 degrees.