The singular form of "aspirin" is "aspirin." It's one of those words where the singular and plural are the same!
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "that" is "it."
The singular form of "cuffs" is cuff.
Aspirin is acetylsalicylic acid. Consequently, it can act as a Lewis acid. Ammonia is a Lewis base. Therefore, ammonia and aspirin can react to form the ammonium salt of aspirin.
The singular form of "stratum" is "stratum." The word does not change in its singular form.
The singular form of feet is foot.
The singular form of "oases" is "oasis."
The singular form of "wash" is "washing."
The singular form of "dice" is "die."
The singular form of diagnoses is diagnosis.
The singular form of scissors is "scissor."
The singular form of vixen is "vixen." Both the singular and plural form of this word are the same.