Q: The profit on our current home is about $80,000. In 1979 we intend to purchase a new home and make it our principal residence. We intend to rent out our old home. What are the facts about the $100,000 tax-free profit which we should be aware of?

A: Under the Revenue Act of 1978, if you're 55 or older you can exclude from gross income (on a one-time, elective basis) as much as $100,000 of any gain from the sale of your principal residence. If it is owned by you and your husband, and if either one is 55 years old or older, that's enough to qualify. To qualify as a principal residence you (or you and your husband) must have owned and occupied the house for a total of three out of the five years preceding the sale.

The one-time election may be made for any sale or exchange effective after July 26, 1978. It must be made in accordance with regulations prescribed by the Secretary of the Treasury. At this writing, to the best of my knowledge, the Secretary of the Treasury has not prescribed rules for making the one-time election.

The act provides a special transition rule for persons eligible under the old law governing non-recognition of certain gain for persons 65 or older. The act also provides that gain realized on the sale of your principal residence after July 26, 1978 is not an item of tax preference under the minimum tax.

The act doesn't affect the law that allows non-recognition of gain on the sale or exchange of your principal residence (defined differently) if a new principal residence, at least equal in cost to the adjusted sales price of the old residence, is purchased and used by the taxpayer in the 18-month replacement periof (24 months if you build a new home, four years for members of the armed forces). For further details, consult your tax adviser or the Internal Revenue Service.

Q: I'm considering putting some money in a syndicate that invests in real estate. Can you tell me what I'll be getting into?

A: I can't tell you specifically, but I can tell you generally what a syndicate is and how it operates.

It usually takes the form of a limited partnership or a joint venture. These should be established by a written document. It should be carefully and specifically worded to establish rights, obligations, benefits and liabilities of each general and limited partner or each trustee and beneficiary.

The owner and manager of the real estate in a limited partnership is usually a general partner or partners. In a joint venture it is usually a trustee or trustees. You are almost always a passive investor whose liability is limited by your investment. So are your benefits.

Under certain circumstances (too detailed to enumerate here) the syndicate must be registered with your state securities commission. Also, in certain circumstances, it must be registered with the federal Securities and Exchange Commission.

There are many ramifications to most syndicates. If you're investing more than a nominal amount, you'd be wise to consult a real estate investment counselor, tax advisor or tax attorney.