Can I purchase a life Insurance policy for my mom (78) with dementia?

Yes, you can purchase life insurance for your Mom but she would be placed in the "high risk" group, thus the premiums will be higher. Call around to the companies that sell mainly life insurance. In your Mom's case, she would be required to have a physical and questions will be asked about her dementia. Watch out for limitations, such as if your Mom should pass within 6 months of getting the policy, the insurance company might not pay out.

It might be easier to purchase a funeral policy instead.